Sometimes, Lightroom doesn’t pick up the install so you will need to add it manually:
Go to Edit -> Preferences -> External Editing. The second area down, Additional External Editor where you will find a line, Application with a choose button on the far right. You will find the executable for Portraiture 2 Plug-in for Lightroom (Portraiture64.exe) located in C:\Program Files\Imagenomic\Portraiture 2 Lightroom Plug-in\x64 Once you have chosen Portraiture64.exe, make sure to change the File Format to TIFF and set Compression to None on the same screen.