As the title say's on climbs the front end tend's to pop up should I put a longer or shorter stem on?
Longer.
longer innit - weight further forwards.
Check your saddle position too - you may be too far back. you want your knee joint positioned vertically over the ball of your forward foot when the pedals are in the 3 and 9 position, and you're sat in your normal riding position on the saddle. Also remember to move forward on the saddle when climbing.
Bent udder, its just about you and me only, that seem to setup that way on here..
Santac, do you drop your elbows on climbs? makes a huge difference to climbing.
Yup, weight forward. Get the horn of the saddle up your primary orofice and get your elbows below the bars so you can pull down
Check your saddle position too - you may be too far back
Spot on.
you want your knee joint positioned vertically over the ball of your forward foot when the pedals are in the 3 and 9 position
That's nonsense.
Front end popping up is 100% to do with seat angle though. So slide that saddle forward. Dropping the fork travel will also fix it. Longer stem, actually less so. Also, sliding the wheel back in the dropouts (if you have adjustable ones) will work too.
5mm adjustment at the wheel is the same as about 15mm of movement of the saddle, in terms of COG movement relative to the rear wheel.
you want your knee joint positioned vertically over the ball of your forward foot when the pedals are in the 3 and 9 positionThat's nonsense
Its only a rule of thumb, its a bio mechanics point of view, we all have very different in torso/limb lengths so its not a definate. Just a decent place to start if you are experiencing, problems with your body, or control on the bike.
As Brant says, bar height and saddle position make the real difference.
I've gone from a 90mm stem to a 50mm one on one bike, and providing I wind the forks down, it still climbs exactly the same.
And as others have said, the steeper it gets the further forward you need to sit on the saddle. A good chamois should mean that the nose of the saddle doesn't get *too* intrusive.
another good tip:
instead of holding the bars with fingers above and thumbs below (ie normally) on steep seated climbs move your thumbs so they are also on top of the bars - so you are sort of hooked over the top of them (non opposing grip I suppose).
This automatically pulls your elbows in and down so when you then pull on the bars the direction is pulling them downwards in the direction of your crotch as opposed to towards your chest when they are 'normal', with the effect that the front end stays down and wanders less.
Try it. It really works, far better than a words description ever can get across.
After sifting through this lot I fixed my climbing problem.
Saddle forward c10mm, body position/shape.
http://www.singletrackworld.com/forum/topic/wandering-all-over-the-shop-when-climging
Rotating your hands\dropping your wrists so your knuckles come back towards you helps you drop your elbows, helps when pulling on the bars as you're pulling horizontally back towards you rather than up.
Get heavier bars 
Being smoother in how you pedal is probably all you need to do. Try to consciously not bob with each stroke.