The swingarm is a Honda type setup. When you loosen the axel bolt, there are two threaded rods that will force the wheel assembly back in it's slots.
If you adjust these as far back as you can go, and still not have the correct tension on the chain, then it's time for a new chain and sprockets.
The back nuts are the ones that will take the wheel to the rear when you tighten them. Loosen the back nut, then use your fingers to move the inside nut out of the way while you adjust the tension with the rear nuts. Once you have the adjustment proper, simply tighten the indie nut back up as far as you can with your fingers, and then do the tightening of the whole thing with the rear nut (which you can get your spanner on).
You need to be aware of what correct tension is.
You want the shock compressed until you have the front sprocket center-swingarm-rear sprocket center all in a straight line (or as close to this as the suspension lets you obtain). This will have the chain in it's tightest possible position...and at this point is where you should still be able to move the chain up and down about 1-2cm. The chain should not be bound at all when it is in it's tightest position.
Many times this will leave the chain looking very loose when the bike is in it's normal postition, yet tightening it up any from there can spell a fast demise of your chain, sprockets, and bearings.