The popup feature in dnn600 is based on a client side javascript call - as such it needs to be triggered from a client side onClick action.
To make this work you would need to change the way your module works a bit.
this is the basic structure of how the call needs to look:
<a onclick="return dnnModal.show('path to your module.aspx&popUp=true',/*showReturn*/true,600,950)" href="path to your module.aspx">Login</a>
the url u pass to this module needs to point to your module with a mid= and/or ctl= parameter - that will ensure that ONLY the module and not any other modules on the page are loaded into the popup.
Alternatively - for example this is a sample of how the HTML module Edit Content action causes the edit content to open in a popup window.
<a href=" if(dnnModal.show('http://dnn600.local/AppleSkin/tabid/102/ctl/Edit/mid/444/Default.aspx?popUp=true',/*showReturn*/true,550,950)){__doPostBack('dnn$ctr444$ctl00$actionMenu$ctlactionMenu','3')};">Edit Content</a>
wESTA