I tend to use the old fasioned way of getting it running.
Place the wide cleaning end in the tank, making sure that it won't come out.
Place a bucket or similar container a few feet below the level of water in the tank. The lower the bucket (actually the other end of the cleaner) the faster the flow of water through the cleaner.
Suck on the narrow end of the cleaner (the flexible tube end).
When the water passes the highest point of the tubing it will naturally start running down the tube faster, towards your mouth! Before the water gets to you quickly stop sucking and place a thumb firmly over the end of the tube.
Direct the end of the flexible tube into the bucket and release your thumb. The water should continue down the tube and out into the bucket at a rate that's dependant on the level difference between the surface of the water in the tank and the end of the tubing.
If the water trickles back up the tube and into the tank then you stopped sucking too soon. Try again!
BE WARNED!
A mouth full of aquarium water is not just an unpleasant experience, there are some fish diseases that can be transfered to humans, most notibly TB. Be very careful or use the alternative method below.
Place the entire cleaner in the tank and completely fill with water.
Place a thumb over the flexible hose end of the cleaner and lift it out of the tank.
Aim the flexible hose into a bucket, holding the end of the hose below the level of the aquarium water surface. Release your thumb and the water will start flowing immediately. If it does not then there were too many bubbles in the tube or the wide end of the cleaner has somehow moved out of the aquarium (start cleaning up now!)
Assuming that the water is flowing nicely, keeping the end of the flexible tube in the bucket, place the other end of the cleaner into the gravel repeatedly to suck up the muck in it. The gravel will be sucked partly up the wide tube but should fall back due to it's weight. If it goes all the way up the tube raise the bucket a bit, if it hardly moves then lower the bucket a bit. Stop the flow every now and again (place thumb over the end) to empty the tube of gravel completely. carry on until your gravel is clean.
KEEP AN EYE ON WHERE YOU ARE AIMING THE WATER AND THE LEVEL IN THE BUCKET!
When you want to stop the flow either place your thumb over the end of the tube (temporary) or lift the end of the flexible tube above the level in the tank (with your thumb over it). Release your thumb and the water will flow back up the tube and into the tank. To start the cleaner again repeat the priming process above (which ever you prefer).
Hope that helps!
WK