TidBITS has a good short article on how to reformat an external drive.
As expected, the instructions that come with hard drives are geared towards PC users. I can never figure out whether manufacturers really don't know that things are that complicated for the Mac, or they just can't bring themselves to admit we don't need their help, but Mac instructions on these things are usually multi-step and include installing un-needed software.
All you need to do is wipe the drive. Instructions below:
How to Reformat a New External Hard Disk