Uninstall your mobipocket reader from your device and reinstall this version: Mobipocket Beta program - eBook reader for Blackberry
You should also make sure your connection settings are correct:
In the BB's Main Menu, enter the Options section.
Then go to the Advanced Options. Choose Applications.
A list of installed apps show, select the Mobipocket Reader with your cursor and open a menu.
Choose "Edit Permissions"
You will then be shown a new section with the permissions of the Mobipocket Reader.
Select "Connections", open a menu and choose "Expand".
Repeat this for "Interactions" and "User Data".
This will let you see how the different connections are setup for the Reader.
It is best (if you can modify them) to set them all to allow.