First make sure that is isnt already formatted in MacOS Extended, connect the drive to your computer, and when it mounts on the desktop ctrl+click (or right click if you have a mouse), and select get info.
Look down the list of informations until you see "format" and it should tell you what format it is. If it is NOT MacOS Extended, you will want to format your drive:
To format the drive you need to go to disk utility
applications-->utilities-->disk utlity
Once opened, select the LaCie drive and then go to "erase" (Formatting will erase your disk, so backup anything thats on the drive to your mac while you're formatting).
Select MacOS Extended under "volume format", and you may want to give it a new name, too.
Click "erase" and it will format your disk.