Hi,
is it possible to create a if/then-Scenario with two different keystrokes within a macro?
More specific:
In Media Composer I want to automatically set transcribed markers (I accomplished that part)
After each marker I want to play the footage at that marker until the User is pressing 'Y' then the macro goes on or 'N' then it deletes the markers just set and goes on.
I played around with the 'Pause until'-Action but was not sucessful.
Any ideas?
Did you explore using a "Prompt for User Input" action? It supports setting custom key shortcuts using /«character», and you can get then access which button the user selected via the %PromptButton% token.
action:Prompt for User Input [Keyboard Maestro Wiki].
Something like this:
Since you want to prompt the user for some input -- how about the Prompt for User Input action? 
Yes-No demo.kmmacros (6.1 KB)
If you want to give them a choice without a prompt (how will they know what/when to choose?) you could probably do something like this:
Pause Yes-No demo.kmmacros (7.3 KB)
...but, unlike with the prompt action, there's no direct capture of which button they pressed -- you are relying on it still being down when the "If..." action executes. It almost certainly will be, but it wouldn't be wise to depend on that in something mission critical...