Turn the bike upside down so that it is resting on its handlebars and on the saddle.

Then take the upper run of chain ( as it is now) in your hands and pull against the spring of the chain tensioner. This will give you some slack chain. Put the chain onto the front toothed wheel (the chainwheel). It will be trying to escape backwards because of the spring.

When you have about half of it on the teeth, if you are on the same side as the chain, gently turn the pedals clockwise so that it moves all the way onto the chainwheel. Once it is on, you should be able to smoothly turn the pedals in either direction and it should stay on.

Check the picture I posted above to see that the rear of the chain is running properly on all the cogs.

When you turn it in one direction, the rear wheel will spin.

Get back to me if you have more problems.

Enjoy your riding.