You need to push the big loop downwards and hold it while with a finger get the catch and hook it onto the top of the trigger which is on the top of the trap. Be carefull with your fingers though as if your not used to doing it you could have a nasty accident. I will post a picture if that's not very clear.
As to placing the traps it's best to leaves them outside for a week or two to weather and lose their smell, your hands should also have a nuetral smell, no smelly soap or oils rubbing them in your compost is a good way to clean them. You need to find a run not a mole hill, they usually go between two mole hills though, you can spot them by looking closley at the grass or there will be a ridge if the runs are shallow. Dig down into one, I use my hands as the ground is fairly soft here and it causes less disturbance to the tunnels, place the trap in and covers with completly with the grass you romoved so that no wind or light can get in, but dont make it to heavy or the trap will struggle to kill the mole, also lay a coloured clothes peg on the ground so you reember where they are as it can be hard when you place a few.
Check traps daily, and be careful removing them as one side of the trap will still be active, to disarm just hold down the loop and remove the catch and release pressure slowly, disarm them all when you have finished and leave out for another week to lose the smell again and then store inside so they don't rust.