You should be supplied with a 90 degree metal corner piece which screws into your dishwasher (see below). If this piece has not been supplied, you will need it, so a trip to the local hardware store will be in order - don't worry though, it should cost under £5. Read through the rest of the installation guide before you make the trip - you may need to buy some other bits and pieces as well.
The important thing with this stage is to remember to use Teflon tape. Basically, you need to wrap one end of the 90 degree piece with Teflon tape, this might seem unnecessary, but it will prevent your dishwasher from leaking, as it creates a tight seal when the piece is screwed into the dishwasher. You can get this tape from any decent hardware store, and it should only cost around £2 - definitely worth the money to stop your kitchen flooding. There are other methods of ensuring a good seal, but this way is the cheapest and easiest.
So, the tape needs to be wrapped round the threaded part of the fitting clockwise as you are looking down the hole of the piece. If it is wrapped the wrong way, the tape will unravel as it is screwed on, so it is important to get this part right. Two full wraps around should be sufficient - don't go crazy with the tape, or you won't be able to screw the piece in!
Wrap the end to be screwed into the dishwasher with the tape (clockwise). Once this is screwed in, simply connect the water supply pipe onto the other end of this 90 degree piece. Simple!
At this point, it's best to reconnect the water at the shutoff valve and turn on the water, just to check that you've created a watertight seal.