I made some progress...not sure if it applies to anyone else....
Original .950 issue: clean install on new Win-7 64 bit box. Installed .77 and it ran ok. Then tried to update to .950 and the install mocked me with the same "not found" message as the OP. Same .915 update worked on the ancient XP box that had gone thru all the updates from the beginning of the beta.
Then today - tried the .951 update and install failed with the same message.
Then installed .915 sucessfully; then installed .951 ok !!! Running it now.
So, .950 & .951 seem to require something that is included in .915 but not in .75. So the newer updates are not fully cumulative, although I thought they were supposed to be.