Facebook changed its API and is no longer allowing Pages with fewer than 2k Likes to install custom tabs.
If your Facebook Page currently has less than 2,000 followers, you may not be able to add POWr to your Facebook Page yet. You will see the following error during the install steps:
Once your Page has enough Likes, you should be able to add any POWr plugin to your Facebook Page.
Please note: It's currently not possible to install on Facebook Profiles.