I am using a Spanish magic keyboard with a macbook air, but i don

Hold down on the key for the vowel you want to stress and you should get a popup with the accent options. type the number below the accented vowel, or use the mouse to select the accented vowel.

You can also, change your Mac's input layout to match the keyboard and then use the accent key to add an accent to the following vowel you type.

click here-> Write in another language on Mac - Apple Support

saraaaaaai wrote:

I am using a Spanish magic keyboard with a macbook air, but i don't know how to stress vowels.

Thank you.

In the picture below, the keys over at the right edge with an orange line around them are used to add accents by typing them just before the vowel.