I don't understand (issue number on mastercard)?

I'm ordering a few books , and I'm using a mastercard to pay for them.

I've tried twice now , and i'm going to ask on here before i try the third time incase they think im a fraud or something.

So here's the problem , well two infact.

1) issue number.. I'm ordering from waterstones , it has a * by it , meaning i have to fill it in... so do I just put 0 in it?

2) on the back of the card , for my security number , I know it's "the last 3 numbers" howeeeever i'm having problems.

there's the bit where I've signed my name , and on that there are 4 numbers. Behind that (sorta looks like the one with my name is stuck on to it , although it isnt) theres a thinner white strip with 3 numbers on that...

Which one do I use?!

As I said i've got it wrong twice , so now im just a bit confused.

Thank you xxxx