AD Forest trust - assigning access via groups

We have a 2-way forest trust setup between forest A and forest B. We are having issues assigning permissions from forest A to access resources in forest B. The attached URL should detail the issue in detail.

I cannot understand why Scenario 1 does not work whereas Scenario 2 does ?

I believe Scenario 3 is the Microsoft recommended way to assign permissions via a trust.

http://i57.tinypic.com/1ier1v.jpg

Thanks