Hi buzzkill85,
welcome to the forum.
Are you sure you haven't messed up things by installing over another mod/patch, or using an old savegame?
And which answer did you give to the prince before you left the tower? If it was not the one on the lines of "I'll go there at once", return to the tower and talk to him again.
If that does not solve the problem, try the console command: G.Museum_Open=1
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