Frances, until Yosemite comes out, it's not possible to use Airdrop between iOS and an Apple computer. You can use it iOS to iOS or computer to computer only at the moment.
I agree it's frustrating since Airdrop shows up on both your iDevice and the computer and it LOOKS like it should work, but it doesn't. (I know - I tried a few days ago, and couldn't figure out why not until I Googled and got to the Apple doc Jim links to first)
"You can't use AirDrop on iOS to share with OS X users, and OS X users can't use AirDrop to share with iOS devices." - the line just above the image on the KB article is the key one, easily missed...
The good news is that Yosemite is going to be out soon - latest rumours say on or right after the Apple event on October 16.
I have the beta of it, but have been too darn busy to install and try it.