AirDrop is only compatible with the following machines:
MacBookPro (Late 2008 or newer)
MacBook Air (Late 2010 or newer)
MacBook (Late 2008 or newer)
iMac (Early 2009 or newer)
Mac Mini (Mid 2010 or newer)
Mac Pro (Early 2009 with AirPort Extreme card, or Mid 2010)
Navigate to your dock and click the Finder icon. This should open a 'blank' finder window. Press CMDShift and 4, then press space. Click the camera icon in the finder window, a screenshot should appear on your desktop. Upload that here by clicking the small paperclip
It's to do with the way AirDrop works, you don't even need a WiFi connection. The computers detect each other and then they either transfer across an existing WiFi network or it creates one between them. It seems different to the Ad Hoc network stuff so it needs a compatible WiFi card.