Press and hold the colon/semicolon (: ;) key (on the right of l ) then type the "a" and you'll get the accent. It's the same for all the vowels. ά, έ, ί,...οί, εί etc. It should work if not come back and we'll figure it out.
If you are new you may need to access the Greek keyboard in your computer. It's easy here are some links: Click here Another way is to use English characters for the Greek. It's in the same link. e.g. a=a ν=n ς=s. But the whole word should be either all Greek or all English letters.
Read the link and bookmark it for later. Also, be sure to read the Tips&Notes on each page top left. Lot's of information there. Please ask if you need any help.