Translation:Please come up to the 15th floor.
I do not believe it is correct to use 第 in this way. 第 is for events that occur in sequence (my first time going to China, my second time going to China). Things that happen to be physically laid out in an ordinal sequence (such as Building One, Building Two) as well as days of the month (the tenth, the eventh) use 号. The measure word for layers or levels of something (floors of a building, layers of a cake) is 层, but I am not sure whether 层 is actually needed in this sentence.