I used the rotobrush on an object and now want to isolate that object and "delete" the background. I turned on the transparency and set it to zero, and exported it to premiere (i only get the option of exporting it to premiere or flash, nothing else). when I do this, the background is still with the object I used the rotobrush on. How can I completely get rid of the background so I can just superimpose the footage in premiere?
You don't set transparency to zero... that means all of your layer will be transparent.
Don't use File>Export, use Add composition to Render Cue or Composition>Make Movie
Read up on exporting with an alpha chanel
Or, if you have Creative Suite, Save your After Effects project and import into Premiere as a dynamic link.
If you don't know what I'm talking about then read the basics of AE and look at a few of the tutorials. RotoBrush is a little bit of an advanced procedure that takes experience to properly use, making AE deliver a usable product (a movie) is the most rudimentary of basics.