<p>I'm currently working on the <code>Tips.js</code> from <code>mootools</code> library and my code breaks on the line that has those <code>el.$tmp</code>, and console says it's undefined</p>

<p>Can anybody help me?</p>

I'd suggest taking your question and posting it, along with a link to the page to either/or/and:
http://mooforum.net
http://groups.google.com/group/mootools-users/topics
That's the community that swarms with it.
Now as for answering it here - I'd need a lot more information (code example?)