can't install add-on

I see in your screenshot that you have selected the ARM GCC tools. Do you have any other tools selected in App Center or is that the only one?
Could you please attach the error log here? It is the .log file in the .metadata directory within your workspace.

Do you get a similar error if you deselect ARM GCC and select a different add-on, such as TI-RTOS?

I successfully installed Ti-RTOS

ARM GCC still won't install.

Thanks for the pointer as to where the log is, it'd be really helpful if this showed up in the error popup! I was looking in the CCS studio app directory.

This error at the end of the .log file is:

!ENTRY org.eclipse.equinox.p2.director 4 1 2016-07-26 15:44:19.818
!MESSAGE Cannot complete the install because some dependencies are not satisfiable
!SUBENTRY 1 org.eclipse.equinox.p2.director 4 0 2016-07-26 15:44:19.818
!MESSAGE com.ti.gcc.arm.osx.feature.group [4.9.3.2015q3_20150921] cannot be installed in this environment because its filter is not applicable.

!ENTRY com.ti.ccstudio.app.center 4 0 2016-07-26 15:44:20.433
!MESSAGE Failed to install the selected add-ons. installIUs=com.ti.gcc.arm.osx.feature.group repos=software-dl.ti.com/.../

I find this odd because on my other OSX machine has installed GCC into CCS successfully. 

2781.ccs.log

I'm not sure how to "run the main CCS installer"?

is it "install new software" in CCS as in the screenshot below?

if so I don't see how to choose the install folder?

Tim Redfern said:

apologies, I ran the original installer again, that's what you mean

Yes that is what I meant: run the original CCS installer again.

Did that work out ok?

yes, seemed to, thanks