If it's a new HD and hasn't been used with another OS, then you should just be able to install it and then use it. If it doesn't work, you might have to reformat it real quick.
If you're running Leopard, which I am assuming you are, run Disk Utility (in the Applications place in Finder - Finder>Applications>Utilities>Disk Utility)
1. Choose the volume your want to reformat (your external HD). It will be "tabbed" over to the right a bit, under the size of your HD.
2. Click "Erase" on the menu bar.
3. Choose "Mac OS Extended (Journaled)" as the Volume Format and name it what you wish.
4. Click "Erase..." on the bottom right.
When it is all done, it should be formated as "Mac OS Extended (Journaled)" and you can add/remove data at your will.
To make sure that it reformated, just option click the drive icon and click "Get Info". Under "General", the format will be there.
Hope this helps

Worked for me.