Technically, get_cvar_string() doesn't return anything (strings are populated by reference) but I can't tell if that's what you're doing because you didn't provide any code to look at . . .
Have you verified that the cvar has the value in it at all? It looks like AMX Mod X's default admin plugin executes this file in plugin_init() so you shouldn't expect it before then. In fact, you should probably not do anything with your custom code until plugin_cfg() (or the newer version of it). If you're not running the default AMX Mod X admin plugin, are you sure it's getting executed at all?