STEP 1:
Open Central Administration
Go to: Application Management > Manage Web Applications
Click on your web app > General Settings dropdown, select “SPD Settings” and Make sure all are enabled.
STEP 2:
Open your Site Collection
Go to: Site Actions > Site Settings > Site Collection Administration > SPD Settings
Make sure all are enabled.
Or Uninstall the Software and Install one more time, it will work. :)
