Win 7 - 32 Bit
Extract the *.zip file.
Double-click the file "Interpolate.mxi".
It takes a while until a file "interpolate.zxp" is produced.
You will be asked to save the file - ok, but don't change the directory.
Then the Extension manager will start. Click install and choose the new generated "Interpolate.zxp".
Hope it helps.