You can create a new application in any location on your computer.

SImply add a name for your new application, browse to the location you want to use, and hit SAVE and the Session will be created and you will only have to answer yes in Datease and the application will be created.

All the drives in L4D are virtually mapped to avoid long paths which DFD will not handle. B: is just a virtual drive mapped to the catalogue where you application is really located. 

After you have saved the session the RUN button will appear.

Hit it.

Done.

The application is actually located in D:\DOWNLOADS\ROUNDING\ROUNDING but for DFD it will look like it is in B:\