Hi Wolfking60
I'm sorry to hear you're having trouble, the problem sounds similar to what other people have reported before. Could you please try the following:
Remove the PyxelEdit folder located here: "/Users/[username]/Library/Application Support/PyxelEdit"
About the .pyxel assigning, the application has it's own format which is called .pyxel. To be able to open .pyxel files, you might need to assign such files to Pyxel Edit, this however is not important atm. since you can't even get the program running.