Lightroom:
Right click on Image > Edit in > Dfx 3.0
You should then get a message about editing the picture with Lightroom adjustments, etc.
After clicking the appropriate format you should get the option to trial or activate Dfx version 3.0

Photoshop:
In the Filter tab you should see > Tiffen > Dfx v3
Click on the Dfxv3 option to launch the Plug-in application.

Aperture:
Right click on Image > Edit with Plug-in > Tiffen Dfx v3                               
Click to launch the Plug-in application.   
                 

Please also read this KB on installing and activating:http://tiffen.helpserve.com/default_import/Knowledgebase/Article/View/246/1/how-do-i-install-and-activate-the-tiffen-dfx-photo-plug-in