The proper syntax is <MOD-DIAM>, so the fact that it's not showing that isn't a good thing. If you change it to all caps, what happens?
Also, what version of SolidWorks are you on. Seems to me that there was an issue in 2013 or 2014 where people were having that problem. You might do a search as I'm pretty sure there was a solution in those instances.
If you are upgrading from SW2013 or earlier, and bring your settings over from the older installs, SOLIDWORKS may still be looking in your old location in the install folder. As of SW2014, you can now assign your own folder for the location of the GTOL.SYM @ Tools>Options...>System Options>File Locations>Symbol Library File. Pointing SOLIDWORKS to C:/ProgramData/SolidWorks/SolidWorks <current version>/lang/english should get your symbols to appear properly.
the easy way to find GTOL.SYM in your system is to use windows search in your C drive and then copy the the address location