Well, about the masteries, that's a very good idea, didnt see that.
about AD, yes, it was AP, im fixing it :)
about runes, i prefer celerity ones, since im not doing a tank Amumu, but a AP offtank killer, so cooldown reduction fits better, at least i find it better. Mresist.... yeh, isn't great, but isnt low for an offtank build... well, i will put some info in rune section.

About Bbcode, this is my first guide, so i dont know how to use it, and i'm thinking about getting some imgs and learn about that, just give me time :)

sry, forgot to vote...

Nice build u have there! However, I feel that even though you are not going full tank but only an offtank, Greater Glyph of Shielding would still be a better choice since the amount of Magic Resist in this build isnt that spectacular, and this added MR would allow you to last longer in teamfights. Also, perhaps shifting one pt from indomitable to veteran scars in your masteries tree would be better since 30 free hp at lvl 1 sure beats reducing 1 incoming dmg in terms of durability, especially since as u said, "amumu is easily pushed out of the minions"

"Their team have AD carry that is pwning us all : This means you need to build Abyssal!!" Just wondering.. did u mean AP carry? Since in the later part you talked about countering DPS carries. Also, using Bbcode to tidy up the guide and add in some colours would be nice as now it looks kinda messy and hard to read, especially the Items section.
+1 anyways for the good guide :D