Everything I see online just says to select the "ajc" compiler from the list of compilers, but that's not an option for me. I've got javac, jikes, eclipse, and javac-in-process, but that's it.
As it is my project compiles from the command line with maven, but if I type anything in the IDE that uses an aspect, I get compiler warnings.
Make sure AspectJ Support plug-in is enabled in Settings (Preferences on Mac) | Plugins. Follow the documentation for more details.
When using proper Maven project, ajc compiler will be enabled automatically.