Maybe you don't have

mfoss — Mon, 12 Sep 2011 15:47:36 +0000

Maybe you don't have installed i.beam, so you can't run i:im().
How to check if it's available:

(ejabberd@localhost)2> i:module_info(attributes).
[{vsn,[158913296780672535307349608191982573996]}]

Or maybe the window can't be opened because you start it as root. See two solutions here:
http://www.ejabberd.im/node/4790#comment-57529

ejabberd - Comments for "Call to i:im() in application debugger failed"