There's no evaluation here.  Haskell does no implicit coercions for you (with a limited exception involving the definition of numeric literals) so you are passing a Fractional a => a (1/0) when a MyBool is expected, and MyBool isn't a Fractional.  (No coercions means, in this case, that a number *cannot* be used as a boolean, neither a real one nor your alternative formulation, without some kind of explicit coercion.)

How you fix this depends on what you're trying to accomplish.  If you really want to use a division by zero there, you need to use an Integral a => a and coerce it manually:

> myAND F (toEnum (1 `div` 0))

or, if you insist on Fractional,

> myAND F (toEnum (fromRational (1 / 0)))

...and both of these require that you declare MyBool as

> data MyBool = F | T deriving Enum

so that you can use toEnum.

If you're just trying to prove the point about lazy evaluation, leave the numbers out of it:

> myAND F undefined

("undefined"'s type is "a", that is, any type; from this you can infer that it can never produce an actual value, it can only raise an exception, because there is no value that inhabits all possible types.)