I am not really sure what happens when an Adrenal Genestealer rolls a "0" in an attack.

Can someone explain it to me in detail?

Over a month late for an answer, but from what I believe, rolling a "0" resulting in a slain Space Marine means that the Adrenal Genestealer's swarm gets to attack again, this time attacking the Space Marine it is next to now. Or, if say the Space Marine spends a support token to force the Adrenal swarm to reroll, it could be said that the Adrenal swarm instead shifts (by the directional arrows when moving swarms), and instead attacks the Space Marine it is next to now. Those are my thoughts, but I could be wrong, and it can be up to everyone to houserule their decision.

I'm 3 years late to this party, but my excuse is that I only got Mission Pack 1 this morning.

After reading the rule cards I was initially a little confused by the Adrenal's "0 attack rule". But then I noticed the bit about the formation shifting. The formation only shifts if a marine has been slain. Therefore if a marine spends a support token, combat carries on as normal just as if it were a regular Genestealer attack.

If the defending marine should run out of support tokens (or have none on the first place), and is eventually killed by a 0 die roll, then the marine formation is shifted to fill the gap where the dead marine was. After shifting is completed the marine that takes that same position in the formation is now attacked by the same swarm containing the Adrenal Genestealer. This continues until the Adrenal's attack is successfully defended against.

But the important point is that the Adrenal-swarm never moves in this type of attack. It is the marines that move when the formation bunches up again.

Have I got that right?