In Windows you just need to hit the key next to L before the vowel in a Greek keyboard: ; + α = ά. I suppose something similar is the case for Macs. In Android the accented vowel options come up if you hold a vowel key a bit longer, as you would do for numbers. Similarly for iOS I guess? Of course you need to have installed a Greek keyboard first by accessing the settings options of your device.
What I described works in Greek QWERTY keyboards. I suggest looking up the type of keyboard your computer has (the layout of characters, it seems you have a QWERTZ keyboard) and find where the accents are. My computer (also Windows) offers the option to install Greek QWERTY keyboards only, but it may be different with your software version.
Where are your ; and : keys? Next to M? Have you tried using them? Alternatively it might be the ' key on 9. (You need to type something like /9 to get an accent on a vowel on a polytonic Greek keyboard, but you only need to install the monotonic keyboard for this course.)