| You use the melodic minor a half step up because it is a nice shortcut to play the altered scale. C7 #9 #5 is a partially altered chord, so you can use the altered scale in it. The full altered chord is C7 #5 #9 b9 #11, I believe, and it doesn't have a natural five in it (there is no G in C7 alt). So here's the C alt scale:
(notice the use of both sharps and flats -- this is correct)
C, Db, D#, E, F#, G#, Bb, C
Also notice that it shares the same exact notes as a Db melodic minor, only starting and ending on the 7th degree of the scale.
That's why we use the "melodic minor a half-step up" trick. It makes it a whole lot easier to think of an altered scale.
P.S. Here's another cool trick: on a half-diminished chord, let's say C half-diminished (also known as minor b5), use a Db melodic minor scale. So on half-diminished chords, play the melodic minor scale a minor third up from the root. Cool, isn't it? | 
