I remember seeing something but I'm not sure anymore. That your DM needs to match your OS or something?
Except that, I wanna tether with my BB (Yes I have a plan for it) and on my provider web page, they say I need DM 5.0 EXCEPT that I have a 8310 and I'm on OS 4.5
A user yesterday was having the same problem. You can try your carrier's BIS site. They may have a download site. Or, if you know a Verizon cell phone number, (any valid number), you can download it from Verizon's site, Verizon BlackBerry Software Updates. You just have to have a number, the site doesn't use it for any purpose other than verifying you are a Verizon user.