This BlackBerry problem is documented in
this Knowledge Base article from the
BlackBerry Technical Knowledge Center, which you can consult for complete details.
Basically, the Desktop Manager alone is insufficient for installing third-party applications
onto your BlackBerry; you also need a copy of the "handheld code" or "handheld software"
installed on your desktop machine. The handheld code is generally carrier-specific and it's
recommended that you visit your carrier's website to obtain the version optimized for your
device. Here are a few links for your convenience:
Follow their provided instructions to install the handheld code on your desktop. Make sure
you reboot your desktop computer. At that point, third-party software will install successfully onto your handheld.