I have to ask, what did you order ? There's really no such thing other than the drive being pre-formatted to HFS+ format (which you can undo easily enough). I hope this "Mac" version didn't cost more 'cause it absolutely should not.
I didn't know they was a difference either. I noticed it packaged this way at Target as well. This is what I got:
WD 1TB Black My Passport for Mac Portable External Hard Drive - USB 3.0 - WDBFKF0010BBK-WESN from Amazon.
42.53 (+ tax). Not a bad price.
And... worked great. I was able to set it last night and it backed up everything beautifully.