This information applies to versions 3 and newer (Windows only)
If you see a message stating that Nik Collection is not available in DxO PhotoLab, or if the option is 'greyed out', check that the 'display Nik Collection Plugin selector button' option is active in the General tab of the DxO PhotoLab preferences.
Make sure that no antivirus or cleaning program on the system is preventing the software from opening. It may help to add both DxO programs to the whitelist of your security or cleaning software.
If this doesn't help, uninstall the DxO applications using the "Add or remove programs" list in the Windows Control panel. If there are any problems during deinstallation, please use the Windows Troubleshooting tool.
When reinstalling Nik Collection, try launching the installation file via right-click and the "run as administrator" option.