Could I use the 25 dollar plan for AT&T Go Phone on my iPhone 4s that is locked to AT&T?

I believe the plan was 250 mins of talk with unlimited texts and mms, correct? Also how does the 5 dollar add-on for 50mb of data work? Do I have to pay another 5 bucks every month which would make it 30 bucks a month?

Yes you can use that plan (I've used it before) and the $5 data add on is optional you need to have it and it's only 50MB which go by so quickly the only thing I recommend those 50MB is for iMessage