Dimmer Switch question...

I went out to home depot and bought a plug-in dimmer switch (Levitron or Lutron??) as per your advice on here. It seems to be all or nothing - When it's set to its highest i get temps of 100 degrees, and when I set it a notch under that my temp drops to 85-ish. I wanna get it in the 88-90 degree range but can't seem to get it right. Any suggestions or ideas?

Thanks

do you have a voltage meter that you could use to verify that the dimmer is working properly?

Or a lamp? :P