The classified you want to respond to was in Mumsnet Local, which is different from Mumsnet Talk, where you are posting now.
To post on Mumsnet Local, or PM anyone in Local, you need a Local nickname. This has to be different to your main MN nickname (because so many users wish, for privacy reasons, there to be no link between their posts on MN Local and MN Talk).
So all you need to do is go here and give yourself a MN Local nickname. If you're not bothered about your two nicknames being different, you could just add a letter or a digit to your present nickname - so tiggercats2a, for example.
Hope that's clear. And sorry it's all so complicated.
MN Local is relatively new, so some of the signposting isn't as clear as it could be. We're working on that...
How odd. Obviously when you registered/joined MN, it doesn't allow you to use a NN that's already in use by someone else, so everyone has a 'unique' NN. And the 'message poster' box is automated as far as the 'To' and 'From' bit is concerned.
Hopefully someone from MNHQ will be along to help you out, although they might have a skeleton staff on duty for the next few days.
Hi, I'm a newbie and joined to reply to a classified, but when I'm logged in, I select 'message poster', write my message, click 'send' and get an error message saying 'you cannot send using someone else's nickname, but there is no where to write my nickname.