If you use Windows, you can go to Start, Control Panel, Regional and Language Options, Add, look for Spanish, there are many options, I recommend "Traditional Sort".
After you have installed it, you will be able to see "EN" next to the time on the lower right corner of your screen, that means you are using English, you can click on the EN and it will show you the languages you have installed. Click on ES (Spanish) and then start typing. Next to the letter "L" there is the "Ñ", next to the Ñ there is the written stress. You can press that key then any vowel and the vowel will appear with a written stress on it.
Cheers.
(Other sorts of Spanish keyboards have the written stress next to the letter "P").