Looking at my CumulusToolbox (version 0.1.3.34) 'Setup' screen, I used the 'Browse' to search for and find the 'cumulus.ini' file - what I have on my 'Setup' screen now is 'B:\cumulus.ini'.
But, for the 'PHP webtag file' option, I had to manually enter the complete and proper path to the webtag file - what I have on my 'Setup' screen now is 'B:\web\cumulustags.txt' (as explained previously, on my systems (both local and online servers) I have used the file name 'cumulustags.txt' but when you release the webtag file on the Wiki, it was renamed to 'cumuluswebtags.txt').
True, I was not able to use the pull-down menu.
So, an easy solution without the need to edit the registry...