Ahhh I see what's wrong
I tried to reproduce what you were experiencing by going through your first post.
So I typed:
A<enter>
B<enter>
which gave me two entities
Then I typed:
M.aKey 11 A.id
and no fields appeared and then I realized I totally missed that 'aKey' and 'id' are navigators, not fields, like Customer.Order 11 Order.Customer. So then specifying 'id' as a field gives an error as it's not a field, but a navigator mapped onto the relationship like 'Orders' in Order.Customer m:1 Customer.Orders
To create relationships this way, you have to specify the navigators you want to use, not the fields, you specify the relationship between entities, not between fields.
For a m:n relationship in our designer, you need two m:1 relationships btw, not two 1:1 relationships. If you just want to define the two 1:1 relationships and not a real m:n relationship over that, that's fine though, and I'll use that below:
Anyway, to fix it, please do: (first remove the existing entities to avoid name clashes with previous attempts)
M.A 11 A.M
M.B 11 B.M
this gives two 1:1 relationships, but no entity fields.
Then type:
A.Id int *
which gives an int typed Id field in A and marks it as a PK field
It will also create an FK field in M. In quickmodel with 1:1 relationships, the start entity is always the FK side.
Do the same with B:
B.Id int *
and it will create an int typed Id field in B too, mark it as PK and will create an Fk field in M for it too.
To mark the fields in M as PK do:
u M
which will move to the scope M. Then type:
AId *
BId *
which will make the fields in M both part of the PK.