mattallen37 is correct; when you install a version of ROBOTC, you are installing the core program and all of the supported platforms (NXT/TETRIX, PIC/Cortex, Arduino, etc). Then, if you would like to use a different compiler, all you need to do is activate it through ROBOTC's Add License option (Help-> Manage Licenses -> Add License). This way you can easily add new functionality to ROBOTC without having to go through multiple installs; once activated, all you have to do is change your Platform Type inside of ROBOTC and start coding for the selected platform!
We actually have a walkthrough guide on this you may want to check out, it's located here: http://www.robotc.net/wiki/Tutorials/Ge ... ate_Online
Please, do not hesitate to let us know if you have any questions or run into any issues at all.
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