The randomiser works for me but I changed it slightly to test.
Code:
lngIndex = Int((16) * Rnd)
MsgBox lngIndex & vbCr & strQuotes(lngIndex)
It wouldn't work if you had your code in a template because it needs a saved document path which you don't have when opening a new document. It also presupposes that that your document contains the tbRandomQuote object.
Could your issue be due to the timing of the AutoOpen macro actually running prior to the opening document becoming the 'ActiveDocument'?