The Spaniard felt aggrieved after referee Chris Foy awarded the hosts a spot-kick in the 32nd minute when former West Brom man Shane Long appeared to trail his leg in a coming together with Craig Dawson.
Ben Foster saved Nikica Jelavic's resulting effort, only for Liam Rosenior to head home the rebound to put Hull in front.
Soon afterwards, Long doubled the advantage when the visitors failed to clear a corner, while Mel felt that his team were hard done by again when James Morrison went down under a challenge from Jake Livermore in the area 20 minutes from time.
This time, Foy waved away the vociferous appeals, leaving the West Brom boss less than impressed.
"We played well for the first 25 minutes but the penalty changed the game," he said.
"I don't know if it was or wasn't a penalty, but I have one question, why we did not get a penalty from a similar incident involving James Morrison in the second half?
"I think it was a bad decision from the referee.
"In the second half, it's the same situation (as the Long incident) in the other box and the referee doesn't whistle.
"Shane Long is a good professional, a good striker. I think he only wants the best for his team, it's normal."
