Sounds like you already got the screws, but just in case they didnt fit here's what i got. Zinc wood screws, #6 inch and a half long. They worked perfectly for me, it was like 70 cents for a bag of 10, i only used 2, they're solid, i dont think i need to do all 4 corners...
Also, if you get the fan and ac adapter at radio shack you notice they use the 2 prong adaptaplug thing on the AC adapter. You can get a 6" adaptaplug power lead, which is a male adaptaplug on one end and the other is the wire stripped and tinned. Just connect this end to the fan, with butt connectors, solder, or just twist and tape, and the plug end plugs into the AC adaptor. Its a little cleaner and the power lead is only $3. Its in the same section as the AC adaptor in the store.