The platform number is largely irrelevant for RER services. (Just as with London Underground services, you simply follow the signs for the correct line, going in the correct direction. Nobody actually uses the platform numbers).
If you're arriving on Eurostar, simply turn left onto the main concourse and keep walking, while looking for RER signs like this one:
http://www.snowcarbon.co.uk/sites/default/files/metro-sign_2.jpg
Initially you'll just see 'RER' (without any indication of individual lines) but, as you get further along (and go down the escalator) you'll start to see indications as to which lines are served, as here:
http://www.experienceloire.com/France/2011%20latest/tgv_rer_sign.jpg
It's incredibly easy. However, to save queuing for tickets (either at ticket desks or at machines) it's wisest to buy a 'Paris Visite' travel pass at St Pancras, Ebbsfleet or Ashford when you check in.
Chris