Open the Message Center using any of these actions...
select Message Center (⌘2) from the Window menu
select Open Message Center from a device menu
type a HotKey set up in the HotKeys Preferences
Font sizes in the Message Center can be adjusted from the Advanced preferences.

On the left side of the window are the message folders. Drag the vertical line to the right of the folders to resize the folder list.
Folder Selection
Select a folder to display the folder's contents in the message list on the right. Use shift-click or command-click to select multiple folders.
A folder that contains one or more sub-folders is displayed with a disclosure triangle to the left of its icon. Clicking the disclosure triangle will show and hide the parent folder's sub-folders.
When the sub-folders are visible, selecting the parent folder will show only the messages stored in the parent folder.
When the sub-folders are hidden, selecting the parent folder will show all of the messages stored in the parent folder and its descendants.
Manage Folders
The folder options menu is available at the top of the message folder list, or by control-clicking in the folder list.
Use the folder options menu to add or remove message folders.
Messages moved to folders you create are stored locally on your computer.
Edit the name of a folder using the folder options menu or by double-clicking the folder.
Drag folders to re-order them.
Devices Folders
A device folder contains messages stored on a device.
Device folder display options are available in the Message Center Preferences.
You can add, remove, copy and edit messages in a device folder, even when the device is not connected. Any changes will take effect the next time the device's message folder is synchronized.
Device folders are synchronized when the device connects, a message is sent or received, or the number of messages on the device changes.
Moving a message stored on a device to the Trash copies the message to the local Trash folder and removes the message from the device. The local copy of the message can then be erased by emptying the Trash.
Moving an outgoing message to a device folder may cause the date to be lost when the message is saved and re-read from the device.

On the top-right side of the window is the message list. Drag the horizontal bar under the messages to resize the message list.
List columns can be customized from the View menu or by control-clicking on the column names.
A message listed with black text is saved locally or on a device. Gray text means the message is saved on a device that is not connected. Red text indicates that changes to the message have not been saved to the device yet.
Message Selection
Select a message in the message list to view it in the Message Center.
Double-click a message to open it in a new window.
Control-click in the message list to display the message options menu.
Vertical-drag in the message list to select multiple messages.
Horizontal-drag messages to move them to message folders. Hold down the option key when dragging to copy the messages.
Drag messages to the desktop to export them as RTF files.
Use the Delete key or the Move to Trash toolbar button to move a message to the Trash.

Messages can be searched from the Message Center by entering text into the search field in the window toolbar.
When text is entered into the search field, only messages with matching text or contacts are displayed in the message list.
Click the magnifying glass in the search field to display the search options menu. This menu contains recent searches and options for changing the search behavior.
All Folders: consider messages in every folder when performing a search
Selected Folders: only consider messages in selected folders when performing a search
Reset When Changing Folders: clear the search field when the folder selection changes

Due to inherent limitations of the protocol for reading text messages over Bluetooth, outgoing messages stored on a device may have missing dates.
To assign dates to messages stored in local folders (on your computer), select the messages in the message list and choose Change Date from the Messages menu or from the message list context menu (control-click the message in the message list).
Moving an outgoing message to a device folder may cause the date to be lost when the message is saved and re-read from the device.