Softimage Xsi: installing 3.8 functioin into 3.9

I want to get a favorite 3.8 tool into 3.9.

It's WELD+, I have the .exe and the .cus file in their proper folders ( at least I think so )

So what's next? I can't find where to get it added into the menu ( under POLYGON )


Thanks in advance for any help


Gerald