well, this depends on which version of mzscript you are using. to get the effect you are looking for, you need to use a newer version of mzscript, which defines variables like this:

in its own separate file, which you would designate by a *mzVarFile line. to get the systray to recognize the variables as evars ($$ variables) as well as mzvars, you will need to put an include line for the varfile in as well.