Okay. here's my next idea:
What version are you running? How did you get it? have you ever upgraded?
If you didn't install it yourself, then you'll need
this link:
That will take you to RIM's site where you can download the OS .exe file. You're okay to enter the information on the screen, or you can make it up.
Eventually you'll be able to select download
While it's downloading, you can save time by uninstalling the existing version, if there is one. You'll need to go to control panel>add/remove programs. Uninstall any items that read "Blackberry device software v7.x.x for 9850" Do not delete Blackberry device software updater.
Once the file is downloaded you'll need to install the OS by double-clicking on the file. Follow through ont he prompts to install the OS.
Once finished, you should be able to go into DM and find BB Maps as described earlier.
It is possible that this OS version that we just downloaded is newer than the version currently on your device. If that is the case, DM may suggest updating the software. Doing that is up to you. I'm not absolutely positive if it is necessary to update the OS version completely just to install one component of it.
This is a pretty basic writeup off teh top of my head. If you're not comfortable or clear with any of this, I'd suggest check the BB 101 forum threads for help. But if you ahve a proper backup of your device, there's really nothing you can do that will screw up your phone... at least nothing that can't be undone.
Let us know how it goes or if you're totally lost.