I don't have mutch time to look into it, but an you find your anser in this helpfile ?
http://b2evolution.net/man/2004/06/16/plugins_on_off_howto

Well non, it's not that... I had already look into it, and the class files are correctly defined...

This comes up time and time again. There should be a sticky about this. The answer is available via search as well, but:

in plugins/toolbars/_smilies.toolbar.php , line 26 ion 0.9.0.11, set the variable to true like such:

var $display = true;

HTH,
-Matt

Oh my god, I can't believe it was THAT easy... Thank you very much, I had totally missed that part...

Well, problem fixed !