The Diaspora folder should be in your Applications folder, along with your other applications.
And yeah, the game is tough. I helped with beta testing, and I found the game to be pretty hard.
(1) Don't be shy to try playing on Easy difficulty for a while if you need. You can change difficulty in the in-game options menu.
(2) Take a look at Aether's Guide
. It has a ton of useful tips.
(3) Make sure you're getting a decent frame rate. The game can be extremely difficult if you have a low frame rate. You can check your frame rate by going to the launcher's advanced settings tab, scrolling down in the flag list to the "Dev Tool" category, and checking "Show frames per second on HUD". The frame rate (FPS) will then appear in the upper-left corner of the screen while you play.
To get a good frame rate, you may need to turn off some of the graphics settings in the launcher's advanced settings tab, like post processing, soft particles, and FXAA. What kind of Mac are you using?