Adobe have changed the way patterns are installed. If you're having trouble installing patterns in CC2020 or higher, head here for the latest installation instructions.
CS6 - CC2019 USERS.
When trying to install patterns (or brushes) via the Preset Manager, some users may find that the preset file (.pat / .abr / .tpl file extension) is grayed out in the Finder or Windows when you attempt to load the preset.
The Preset Type selected must match the preset you are installing.
Check that the correct preset is selected in the Preset Type dropdown before clicking the Load... button.
Try moving the preset file to a different location on your computer before installing.
This is especially helpful If you previously moved the preset file to Photoshops application folders (Applications (Mac) / Program Files (Win) > Photoshop > Presets).
Once the preset file has been moved, try installing again either via the Preset Manager or by double clicking the preset file.