[kh99]

I can't tell you exactly since it depends on what your plugin does and what your template changes are. But I think whatever variables you are trying to put in the template should be declared as global. For instance if you're trying to display an ad by putting $ad in the template somewhere, then you'd put

global $ad;

as the plugin code. (That is, a new plugin using hook location forumbit_display and only that one line of code).

ETA: if you haven't created a plugin before: Go to "Plugins and Products" in the admin comtrol panel, choose "Add New Plugin". Choose the hook location and make up a title (so you'll remember what it is later). Put the code in the big text area, select "Yes" to make it active, and save.