Add active class to link  using l()
You can't. The l() function is only responsible for rendering the HTML link, it has no concept of the tag it's being wrapped in (or indeed if it's even being wrapped in another tag at all).

The simple answer based on your question is to add it manually:

&lt;li class="active"&gt;&lt;?php print l('Hapiness','my/hapiness/complete', array('attributes' =&gt; array(), 'html' =&gt; FALSE)); ?&gt;&lt;/li&gt;