Hmm, that entry should definitely be there when you have WMP installed. Did you perhaps use some kind of registry cleaner?
To re-add the entry, copy/paste the following into Notepad, save it as a WMP.reg file, and double-click this file to add it to the registry. Then launch WMP Tag Plus setup again - it should now find WMP and continue with the rest of the installation.
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\App Paths\wmplayer.exe]
@=hex(2):25,00,50,00,72,00,6f,00,67,00,72,00,61,00,6d,00,46,00,69,00,6c,00,65,\
00,73,00,28,00,78,00,38,00,36,00,29,00,25,00,5c,00,57,00,69,00,6e,00,64,00,\
6f,00,77,00,73,00,20,00,4d,00,65,00,64,00,69,00,61,00,20,00,50,00,6c,00,61,\
00,79,00,65,00,72,00,5c,00,77,00,6d,00,70,00,6c,00,61,00,79,00,65,00,72,00,\
2e,00,65,00,78,00,65,00,00,00
"Path"=hex(2):25,00,50,00,72,00,6f,00,67,00,72,00,61,00,6d,00,46,00,69,00,6c,\
00,65,00,73,00,28,00,78,00,38,00,36,00,29,00,25,00,5c,00,57,00,69,00,6e,00,\
64,00,6f,00,77,00,73,00,20,00,4d,00,65,00,64,00,69,00,61,00,20,00,50,00,6c,\
00,61,00,79,00,65,00,72,00,00,00