Hello FantasticMissFox. Sorry for the delay in answering your question.
The short answer is that we don't allow nannies/childminders to advertise their services on our main discussion boards or on our Local Talk boards - partly because many of our users don't like it and partly because we were rather being overrun, particularly with "ads" from the bigger nanny agencies.
If you have seen posts like this that break our rules, do report them to us and we'll delete them: we do like to make sure our rules are applied fairly to all.
The good news is, though, there are listing directories in every Mumsnet Local site where you can list your services as a nanny (or childminder).
It's a permanent directory (although you can edit it if your details change), so it's actually a more lasting thing than a post on a discussion thread and will probably be seen by more people.
There is also a couple of rolling discussion threads, like this one in our Childminders, nannies, aupairs etc topic, where MNers post when they're looking for childcare, and we turn a bit of a blind eye to childminders/nannies posting too - as long as it doesn't get too ridiculous (eg repeated postings every five seconds).
But, again, Mumsnet Local is still probably a better bet, as the Childminders, nannies, aupairs etc topic is a national one, and Mumsnet Local is likely to targeted more accurately towards people looking for a nanny/childminder in your local area.
Hope that helps! Do please shout if you have any other Qs about this.