It drove me crazy too. There is bugger all documnetation about IM and blackberry or its well hidden.
The Enterprise messenger client is located in the orginal CD of your BES 4.1.x install or if you go to RIM's website and download the Exchange BES version you have installed its included in the BES 4.1.x .EXE
If your like me and don't have the orignial CD anymore or its an older version just use the one from the RIM website. You'll find the client in the IM directory and its called LCS.ZIP. The other LCSOTA.ZIP is for webserver disemination.
I used this guide to diseminate to all my blackberries: http://www.blackberryforums.com.au/f...e-devices.html
This guide is for Lotus but its the same for Messenger and Exchange.
The wireless deployment is not instant. Mine seemed to deploy the IM clients 1-2 hours later.
Hope this helps,