I see. when you add your custom nsec the default nsec that was generated from your master key is removed. this is what needs to be confirmed.
you have to enter the display name. see here: 
So I've already done the connect a wallet part.....then
https://void.cat/d/6D1iNZMV8sFRenXtt1G8QC.webp
So I put my nsec into that relevant place and then this message pops up...
https://void.cat/d/Q6JTuv82vRkXYqLwVDUXZR.webp
I don't know what the "name" of my account is. Where would I have chosen(?) that in the beginning?
I see. when you add your custom nsec the default nsec that was generated from your master key is removed. this is what needs to be confirmed.
you have to enter the display name. see here: 
Thx. All good now. It was just that "name" that was eluding me.