So I'm probably missing something extremely easy... Someone made a tutorial a while back, and I just followed it. Went to coinbin, made a new address, saved the private key, put the public key on my alphabay account, and here's where the problems begin. Every vendor with a green MS next to their name doesn't have a multisig option upon clicking their listing. Even when I set the search function to show MS only, none of the listings allow for me to use MS.
Halp plox
If the listing has the [MS] next to it, then try and place the order, once the order is made you will likely be presented with a multisig escrow address to send the alotted amount of bitcoin to before the order is sent to the vendor.
EDIT: Seems you might have to fund your account balance first, then send coins from there to the multisig escrow address. maybe /u/alphabaysupport can shed some light.