Hi there, annickjanson,
There are two common problems that people have starting their synths.
Problem 1: "I see the synth button, but it won't let me push it."
Solution: You must give your synth a name (top left hand blank next to the thumbnail) and add at least three photos.
Problem 2: "The 'Synth' button doesn't even appear on my screen!" (Example: http://getsatisfaction.com/livelabs/topics/where_is_the_start_synth_button )
Solution: Your computer monitor must have its resolution set to more than 600 pixels tall in order to see the button. You can still start your synth even if you can't see the button as long as you have given it a Name and at least three photos by clicking in the 'Tags' or 'Description' boxes and pressing the Return/Enter key.