I'm a little puzzled on grab effect, if your attack was successful, does the resist the grab effect get rolled separately or is the effect applied to the initial roll?

In other words if the to hit attack value is 10 and say a 20 is rolled for an effect of 30, but the strength or grab power is 12, is the 12 applied to the roll of 20, or is a separate roll required to confirm the grab and the 12 applied to that?

For a grab it's all separate.

Step 1: Attacker makes an attack check against the target.  If attack is successful, proceed to step 2, if not, grab fails.

Step 2: Target makes their resistance check.

For target this is: D20+Strength OR Dodge (whichever is better)+Any modifiers imposed by GM/Other effects/etc

They're trying to meet/beat a target number of 10+Attackers Strength or 10+Rank of Grab Effect

If the target passes that check, the grab fails.  If the target fails that check, determine the degrees of failure to find out the severity of the grab (restrained, bound, etc).

is it 10+ or 15+ ?

10+ as far as I've always interpreted, since it's a standard resistance check to resist an effect (in this case being restrained, bound, etc).  The 15+ is only used for damage resistance checks I believe.