Probably used the wrong hook. maybe also didnt change the names

(EDIT) I just changed my plugin to global_start and it still shows up fine. as a matter of fact i can now use my $varible anywhere on the board.
first, where are you wanting the sponsor template to show up?
if you want it to show on your forumhome, then you would use the forumhome_start hook and put this code in that hook.
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What this does; Every time the forumhome template is loaded, it executes this code, which loads the template into the $sponsor variable.
Once that is loaded, anywhere on forumhome that it sees $sponsor, it spits out the contents of that variable....
at least i think this is how it works.
im not sure about the global hooks, so ive been using specific hooks.
Also, you may not have set the plugin to active when you created it.
If there is a strikethrough on the title of the plugin, then it isnt active.