Did you look at FAQ 340 on the GTAC website? It's titled: How to use the SRS option in the Conditioning Toolbar?
In rev 17 of LMS Test.Lab, you can search the FAQ's from the Help and Search Manuals, Theory and FAQ menu pick to find the same FAQ 340. It is also described in the Help for Mission Synthesis.
If you search the Help for SRS on the Desktop you also get a description of the SRS command on the Conditionnig toolbar and how to use it.
Here is a new article explaining Shock Response Spectrum:
Might give some insight. Has some animated pictures.