Ha! I found an answer.
Go to RubyMine -> preferences -> Keymap
In the big screen with all the menu items, you can select the menu item you need (in case of folding : Main Menu - Code - Folding).
Rightclick on the item you want, and there you can make your own keyboard shortcut, with keys which are actually on your keyboard ;-) Or even your own mouse click.
Ha! I found an answer. Go to RubyMine -> preferences -> Keymap In the big screen with all the menu items, you can select the menu item you need (in case of folding : Main Menu - Code - Folding). Rightclick on the item you want, and there you can make your own keyboard shortcut, with keys which are actually on your keyboard ;-) Or even your own mouse click.