I think the last thing the OP who seems to have a somewhat limited knowledge of newer macs needs to be doing is trying to install the GM or anything else besides restoring to Snow Leopard.
OP, by your admission....
"I'm not an idiot I'm a woman who knows naff all about computers these days!!
I used to be quite handy on a computer in the 90's but slots changed since then!"
Why are you messing around with beta software? It will be out most likely in the next 7 days. Unless you have a pretty firm grasp on the current generation of Macs and OS X you should not be messing with that stuff.
OP, by your admission....
"I'm not an idiot I'm a woman who knows naff all about computers these days!!
I used to be quite handy on a computer in the 90's but slots changed since then!"
Why are you messing around with beta software? It will be out most likely in the next 7 days. Unless you have a pretty firm grasp on the current generation of Macs and OS X you should not be messing with that stuff.