I'm trying to use a USBASP programmer to upload to my pro mini, but how do I select it as a programmer? I try selecting 'USBASP' under the tools>programmer menu, but the IDE doesn't recognize the programmer.
Works fine for me. I suspect you are doing something wrong. Perhaps if you said exactly what you were doing someone could tell you how to fix the problem.
Select USBASP as programmer, and then to upload, choose "Upload using programmer" (not the normal upload button - if you're using a chip that normally has a bootloader (like a pro mini) and the board.txt entry is set up as if it has a serial bootloader, the upload button will try to use that. You need to use Upload using programmer to use the USBAsp instead of serial.