PCs cannot access other PCs

Forgive me in advance, this is my first advanced router
I have purchased an RB450G

Previously I used a WRT54GL equipped with either dd-wrt or Tomato 1.27 firmware
The router was good, but it would crash sometimes under heavy load and it did not have many features.
After about 5 years I needed a new router
It is by chance I stumbled upon routerboard.com and I picked up a RB450G

I installed Winbox and have figured out how to connect using either the MAC or the IP
It was difficult because the router kept getting 0.0.0.0 and I couldn’t get Winbox to connect, then I read a post that said click on the MAC address instead of the IP and connect to the MAC address instead; this helped me configure my interfaces

I was able to get http://192.168.1.1 working in my browser so I could access the http interface and setup basic things
I am not really sure what some buttons and fields do, I have gone through lots of documentation, googling, and wikipedia

So far I think I have working

  • a dsl modem and pppoe connection so my internet works
  • dhcp server so I can pull IP addresses for my devices
  • dns server / dns cache

I can use Winbox and see all the other PCs that have pulled IP addresses from the RB450G by going to
IP → DHCP Server → Leases

Now: I have a fairly confident feeling that I have screwed up some fundamental basic configuration of the router / switch, so where can I find a completely noob beginning walkthrough. I have tried searching for some basic setup guides but they all end after the upgrading of the routerboard (btw I went from lvl4 to lvl5 by having to drop the new packages into the router using System → Packages)