I've sorted out a few Mahalos - most are quite easily fixable:
1. Tighten the loose tuner. No point going any further until the tuners don't slip. If they are friction tuners (i.e. they stick out at the back) then tighten the screw 1/4 turn at a time until the tuner stops slipping. If these are geared tuners (they stick out at the side) then your string may be slipping through the hole in the tuner post. Unwind it, push string through hole, pull tight, if possible push string through again after a half turn round the post, then start tightening. If the turns of the string start overlapping each other on the post, your tuning will probably slip.
2. Check the 12th fret intonation. Play an open note, play the same note at the 12 fret on that string. Is the note the same (only an octave higher)? If so, great. However, on all the Mahalos I've seen the 12th fret note is a bit higher (sharp). Never fear. Look at the bridge - there is a white strip of plastic set into it, the saddle. The saddle has a camber cut on it, and the sloping side is facing the tail of the uke. Slacken the strings and push the saddle out of its slot in the bridge. Turn it round so that the sloping side is facing the tuning pegs. Re-tune and check the 12th fret - should be closer, and in my experience is usually almost exactly the same note an octave higher. Note: this does nothing irreversible to your uke, you can simply turn back the saddle as it was before if it doesn't improve matters.
3. The nut (the piece of white plastic between the fretboard and the tuners) probably has the slots cut too high. If you fret a string gently at the first fret, and then press harder, does the note change a lot? Thought so. This one is tougher to fix, though - you must either sand down the base of the nut (a gentle tap with a hammer should release it once the strings are off) or use a junior hacksaw blade or a nail file to make the slots deeper. If you mess this up, by (a) sanding too much off the base, (b) cutting the slots too deep or (c) making the slots wider, it's hard/impossible to fix. Only try this if you know what you are doing!
However, fixing these three things tends to make a Mahalo very playable. Assuming you've no set-up experience I'd say fix the tuners first, then swap round the saddle. Leave the nut until you visit a uke clubb where there will be someone who can sort that out for you.