how can access the value of a global variable for a condition/subevent of an event?
If it's global, it appears everywhere. So it should just appear in the 'set/add/subtract variable' system actions.
an i use defined global variables in every event sheet?
Yep, go ahead and try it, it should work just fine. That's the purpose of global variables.
f so, how and shouldn't it be displayed as compendium in every event sheet?
I'm not sure what you mean - they should just appear in the system actions as before. If you don't want a variable to appear in another event sheet, move it in to a subevent or a group and it'll become a local variable. Then, you can only access it in that "scope" (that group or that event's subevents).