This is due to Tasktop Dev for Visual Studio being unable to locate the Tasktop companion application. When started, the Tasktop companion application creates a file called .tasktopHTL which contains information on how Tasktop Dev for Visual Studio can access it to get information about your Task List. This file is generally stored in your home directory (i.e. C:\Users\username or C:\Documents and Settings\username) however in some cases the file is not created in the expected location.
If you are using a version of Tasktop Dev for Visual Studio version 2.0.1 or earlier, you can try adding the following on a new line at the end of the Tasktop.ini file in the Tasktop installation directory (i.e. C:\Program Files\Tasktop):
-Duser.home=<path to your user home> i.e. -Duser.home=Z:\Users\sminto
If this does not fix your problem, please contact firstname.lastname@example.org so that we can assist you further.