TAdams-
You can only use code this:
var value = LEMval('INSERTANS:81754X121X2121');
If you also have something like this elsewhere on the page:
{INSERTANS:81754X121X2121}
or more easily (assuming that 81754X121X2121 is question q5):
Expression Manager finds all variables within curly braces, and creates JavaScript arrays with metadata about them - but only for variables "declared" via those curly braces. So, if you use LEMval('q5') without putting {q5} somewhere on the page, LEMval() will return an empty string since there will be no entry for 'q5' in those JavaScript arrays.