Are there events on the drive already? In other words, in the past, this drive worked fine? In that case, I would suggest that you unmount the drive. Then shut down your Mac. Turn off the drive (or unplug it). Remove the cable to the drive. Attach the cable. Power the drive back up. And restart your Mac.
See if iMovie sees it then.
Other things that could cause this...
1) Drive is improperly formatted. If the drive is formatted as FAT32, iMovie will not let you use it. The solution is to reformat the drive using Disk Utility. You can check the formatting on the drive by right clicking on your drive on the desktop (or in FInder) and select GET INFO. In the Info window, look under the General section, and look at what it says next to FORMAT:
Note that reformatting the drive will erase anything that is currently on the drive.
2) If you have moved the iMovie Events folder somewhere on the drive other than at the top level of the drive, then iMovie will not see it.