I have seen that many people are having problems installing the new messenger via desktop manager, and since the ota is not working at the moment I thought I might start a thread to help..
I have tested this on my 8330 v4.5.0.160os from Cellular South using Desktop Manager v4.7.. So I cannot guarantee it will work on any other versions.. I would strongly recommend upgrading from v4.5.0.77 before installing this application, as I have read that many people have lost their messenger all together when using .77 or lower..
Also, you MUST have your OS downloaded and installed to your pc in order for these directions to work..
***WARNING: DO NOT ATTEMPT TO PERFORM ANY OF THESE ACTIONS WITHOUT FIRST BACKING UP YOUR OS IN DESKTOP MANAGER.. I WILL NOT BE HELD RESPONSIBLE IF YOUR OS CRASHES AND YOU LOSE EVERYTHING!!!***
Plug in your blackberry via USB cable to your computer..
Download and unzip the files needed on to a folder on your desktop..
They are located here:
Blackberry Messenger 5.0
Copy all of the .cod files from this folder and paste them into the folder listed below..
Depending upon where you save your files on your computer and also which version of windows you are using will determine the location of this folder.. More than likely your files will be located in one of the two locations listed below..
C:\Program Files (x86)\Common Files\Research In Motion\Shared\Loader Files\8330-v4.5.0.160_P3.4.0.41\Java
or
C:\Program Files\Common Files\Research In Motion\Shared\Loader Files\8330-v4.5.0.160_P3.4.0.41\Java
It will ask if you would like to replace a number of files.. Click yes for each of them.. Once this has finished you may exit the folder..
Start Desktop manager..
Click on Application Loader..
Click browse..
Select the .alx file located in the folder you unzipped to your desktop..
Your screen will probably say something like "There are no additional applications designed for your device.", but if you notice when you scroll down, your bbmessenger should say "upgrade" underneath action..
Hit next.. let it load it to your phone and reboot.. and VOILA!
You should now have BBMessenger V5.0!!

Yay!
Also... Many people have been asking for the link todownload the zip files for BBMessenger so here ya go:
Leaked: BlackBerry Messenger 5.0 For Many Devices! | BlackBerry Rocks!