Think like a dj which is what dance music is about. Find a groove on the record which has a beginning and a ending. Usually two, four or up to 8 bars. This is the section that is going to drive the crowd crazy like djing on the floor. Now mainly to recognize this listen to the record as it plays most dance records or commerical records have a structure intro, verse, chorus, break, chorus, outro and etc. Whatever part you use you can restructure back into a similar structure so that a dj can play it out too.
Now once you find the phrase you were looking for you can chop it up into mini phrases or notes to your liking a play a new melodies with different sequences intro, verse and yada, yada. Whatever the rhythm you add can be close to the original of entirely new it is up to you.
Main thing is that you are in a sense djing two records your midi sequnence and the looped or chopped up melody you created. Like a dj the two records must beatmatch (same tempo), blend well (pitch), and time signature (groove, swing, and rhythm) to match up unless you chop it up finely and groove quantize it aka record to the same grid.
Check the orginal tempo of the record first ranging from 50 to 120 to decide what kind of dance tune you want to make
House 120 bpm-up
R&B 63bpm and up
Hip Hop 80bpm and up
and etc......
Just some rough guidelines nothing exact and retry remixing your favorite artists accepalla to get an idea of were you want to go.
Listen to records and break it down in your head but learn the gear first then it becomes easier cuz you will understand what the mofo did on the record.
Also you should go here too.
http://www.mpc-forums.com/viewtopic.php?t=48101
http://www.mpc-forums.com/viewforum.php?f=18