This seems to be a bit of a mystery. Microsoft uses the initials MUI for its "Multilingual User Interface Technology", but clearly that is not what this registry key is for.
There's some information on the MUICache here;
Follow it through, and you will see a fine description of it by the poster "inbfedsp", about 2/3rds of the way down.
As to your problem I am not sure. The MUICache entry should, it seems to me, be made every time you open that Poivy application, but as it is not doing so, I wonder if the application has faulted, or become corrupt in some way. Have you re-booted the computer and tried again? It may be a temporary glitch.
If not, I would suggest you uninstall the application, re-boot, (restart), the computer to update the registry, then try re-installing the application again.
Sadly it seems that Poivy does not have any FAQs or a Forum, where you might have been able to find out more, but they do have a contact option under their Feedback section;
Hello everyone, I have a few questions concerning the MUI Cache.
I have been running a program called Poivy on win XP, it was working fine this morning and suddenly this afternoon I have this error coming up.
" missing mui poivy"
When I try to open it it is leading me to the internet, instead of opening the program. I cannot get the program to open.
I have had a look my Hijack this log to see if there is a virus or if any settings have been changed. I have noticed some new entries...
What is the MUI Cache? I have looked in the registry and I have 240 entries here. Is this something I can delete, is there something imparticular I should be looking for in the MUI cache?
Thanks for your help!