Assuming the disk has been formatted and attached to the Express, open Airport Utility, select the Express and click "Manual Setup". You will see "Disks" in the toolbar. Screen shot 2009-11-19 at 15.37.13 .png
"not all those who wander are lost….." j.r.r. tolkien
I note with interest that the drive in question is formatted "Apple Partition Map," which is used primarily by pre-Leopard, pre-Intel Macs. You may wish to reformat that to GUID, particularly if you ever want to use that drive as an Intel-bootable backup.
The iMac that I am connecting it has Leopard, the other Macbook Pros all have Snow... my question Rui is; how can let's say computer B & C connected to the external hard drive on computer A?
Don't I need to go to "GO/CONNECT TO SERVER" and then put the ip address of the iMac?
On the imac go into your system preferences, and then click 'sharing'. in that preference pane you're going to have to check the 'file sharing' box on the left. If you want everyone on the network to be able to access the drive the easiest way would be to make the drive a public folder. In the 'public folder' box click the '+' button and select the external. You can also set up the drive to be only accessible by certain users, and password protected....but thats a bit more involved and i'll leave it to you to research that.
Now when you're on the macbook pros (as long as they're on the same network) you should see a 'shared' section on the left side of expanded windows in the finder. Under the shared tab you should see the imac (or whatever the computer's name is).