If cash is tight and you can't get the thumb thing to work, you can buy a clip for your vinyl hose at lhbs for < $1.00. Put that at the low end of your tube, clamp it, and sanitize it. Then fill the tube with water as before... when you have the free end of the hose in your wort and the clamped end in the fermenter, you just unclamp it and it should start flowing... if not, there is not enough vertical distance between the two or you did not use enough water.
A good thing to do is to practice with a couple jugs of water in your kitchen before you do it with beer or wine. That's how I learned... and have never had any problems. Later, I inherited an autosiphon and admittedly they are awesome and really the best way to go. But if that's not possible you can still make life relatively easy for yourself.