Here are some things to check:
Photoshop has not yet been restarted.
No image is open.
You have an old version of the plug-in. Replace it with the current version from
here.
You have a Windows computer, but downloaded the Mac version, or vice versa.
You downloaded the correct thing but didn't expand open the .zip file / open the .dmg file.
You have two copies of Photoshop on your computer. The plug-in is correctly installed in one, but you've launched the other.
You have two different versions of the same plug-in installed. One of them is old and incompatible; Photoshop is trying to use that one and ignoring the good one.