You have to indent the guards past the declaration of your function:
myMax a b = a + b - myMin a b
where myMin x y -- Also, use different variable names here to avoid conflicts
| x < y = x
| otherwise = y
Or as
myMax a b =
let myMin x y
| x < y = x
| otherwise = y
in a + b - myMin a b
If you're using tabs for indentations, I would highly recommend to using spaces, they're less ambiguous with Haskell. You can use tabs, but I see a lot of people run into parse errors because of it.