When a cleric meets an undead creature that is beyond his/her powers to turn/command, then the cleric must roll--on a d20--a number equal to or greater than the Hit Dice of the undead +1/level the cleric is deficient of being able to affect said undead species.
For example, a first level cleric encounters a vampire: HD 9 (I think) and unaffected by clerics less than 6th level,* 9 + (6-1) = 14. Therefore Archie the Acolyte needs to roll a 14 or higher or be turned/commanded by Count D. This Rule also applies to Ponce the Paladin.Me neither.
* Crunchier AD&D folks might notice a discrepancy in these numbers from the AD&D standard--or they might not, I really can't say. If there is a discrepancy it's 'cuz I'm referencing the HackmasterPlayer's Handbook "Table 12K: Turning Undead" for the undead turning probabilities shown here because, well, the Hackmaster books are closer at hand. I mean c'mon--my friggin' AD&D PHB is all the way over there on that bookshelf.