DM 4.7 was designed for the Storm, and works on the Bold and 8900 as well. There have been a few problems using DM 4.7 with the older BBs like the Curve and Pearl.
a. Uninstall the previous version of DM before installing the new one?
b. Restart your PC?
I have seen this before, and I suggest uninstalling DM (all versions if there's more than one installed), restarting your PC, downloading DM 4.6 again and installing and restarting again.