Run the runme.sh or runmeinwindows.bat, depending on your platform.

If they don't work, try running "java -jar ./Convergence.jar" from the command line.

Tips:
Your amount of blocks on the left hand side is your health.
If you lose all your health, the game will pause until you gain some more (you can't lose, though the game may crash at some point (that is a kind of losing)).
Match up different blocks side by side to improve your capabilities - place a damage enhancer (green sword) or rate of fire enhancer (green dots) next to a gun block to fire better. Avoid putting blocks that damage your gun's capabilities near it!

Bugs: if you do too well (and fill up the left hand side), the game can go a bit crazy and crash hard.
If you last for a long time, you can move faster than your bullets, so they just hang in front of you as you shoot them.