charizard,
Ok, now I see the attachment...
Seems to me that this is a two part model. (white and Green). I see in the green model 4 bolt holes. Do those holes are used to attach the green part to the white one? If so, then this is all you need to care about in terms of attachment.
As a start, you can use RBE3 elements that link all the hole interior nodes of the holes to one nodes on the white solid. Do this four times to represent the four bolts. This is a crude way of modeling fasteners, but if you dont really care about the fastener loads an/or the stresses around those hole, that will do the job. You can look into CFAST as well, could work.
Then if you are telling me that the green part and the white part are in fact one and only one part, then glued contact could be the "fastest way" to attached them together.
It all depends on what your are trying to achieve in the simulation.