I need to move all my messages in Mail from my iMac to my MacBook Pro. How do I do this and be sure I get all my recent messages on to the new computer?
There are a couple of ways you could go about this. If you're specifically interested in your recent email and not the stuff you've been hoarding since 2002, it makes sense to create an archive of just those messages you want and then move them. Like so:
Launch Mail on your iMac and from the Mailboxes menu choose New Smart Mailbox. In the sheet that appears create a smart mailbox with a condition that reads Date Received Is in the Last 6 Months. If you'd also like to include messages you've sent, enable the Include Messages From Sent option. Give the mailbox an intuitive name such as Recent Email and click OK. A Recent Email mailbox will appear under the Smart Mailboxes heading in Mail's Mailboxes pane.
Now Control (Right) click on that smart mailbox and choose Archive Mailbox from the resulting menu. Choose to save the mailbox archive to the desktop and click Choose. A Recent Email.mbox item will appear on your Mac's desktop. Copy this item to your MacBook Pro.
On the MacBook Pro, launch Mail and from the File menu choose Import Mailboxes. In the window that appears be sure that Apple Mail is selected and then click the Continue button. Navigate to the Recent Email.mbox item and click Choose. Your messages will be imported into an aptly named "Import" folder that appears under the On My Mac heading in Mail's Mailboxes pane.
If, on the other hand, you want an exact duplicate of your email from the iMac on your MacBook Pro, that can be done as well. Quit Mail on your iMac, move to the Finder, and from the Go menu choose Library (if you're using Lion, hold down the Option key and click on the Go menu, the Library entry will appear). Locate the Mail folder inside the Library folder and copy it to your MacBook Pro. Before leaving the Library folder, open its Preferences folder and copy the com.apple.mail.plist file to your MacBook Pro as well.
On the MacBook Pro navigate to that same Library folder and move its Mail and com.apple.mail.plist files to the desktop for safe keeping (you'll likely be prompted for your Administrator's password). Copy the versions of these items from the iMac to the same location on the MacBook Pro.
Now launch Mail on the MacBook Pro and your should find that it contains the same messages and mailbox structure as Mail on your iMac. If it doesn't, you can put things back the way they were by removing the items you just added to the Library folder and replacing them with the original items you moved to the desktop.
There's a good chance that this won't work if the name of your user account differs on the two Macs--on the iMac that account is called "Wally" and on the MacBook Pro it's called "Wallingford," for example. This is almost sure to trigger permission issues, but those issues can be addressed.
Select the copied items, press Command-I to bring up the Info window, and take a gander at the Sharing & Permissions area of the window. If the current administrative user doesn't have Read and Write privileges you'll want to assign them. You do that by clicking the Lock icon, entering your Administrator's username and password, clicking the Plus button on the left side of this area, and from the list that appears, adding the administrator's name for this Mac. Then give this user read and write privileges. In the case of the Mail folder you'll want those permissions to apply to everything in the folder. To do that click on the Gear icon and choose Apply to Enclosed Items. At this point, everything should be hunky dory.