Hello! Assuming you are running OS X 10.6 (or even 10.5 I think) do the following...
ON the iMac where the External Drive is
From the Apple Menu go to> "System Preferences"> then go to "Sharing"
Check the box that says "File Sharing" on the left hand list
in the middle list that says "Shared Folders" Click the "+" button and find the External Drive, select it (or select the folder(s) you want to share) and add it to the list.
Click the "Options" button and make sure "Share Folders using AFP is checked (to share to another Mac) to share to a Windows machine Select the share with SMB also.
Make sure the user account you want to share via is checked as well.
Now on the Macbook, while you are in the finder (at the desktop) select from the menu bar "Go" then "Connect to Server..." if you know the IP address of the iMac (if it is static) you can put it in, otherwise click the browse button and find the iMac then drive you want to access, you may need to put in the user/pass from the iMac, the one you checked under the "Options" button in the above steps.
If you need anything else, or have trouble let us know...