Yes, the wireless interface will need to be on a different ip range to that recieved from the modem. Your D-link wireless router is no doubt recievinf a 10.x.x.x/x IP from the modem and masquerading traffic onto the 192.168.x.x/x network that your pc is being given from the d-link box.
This is infact exactly what you are trying to achieve with the RB153, but with hotspot enabled
Does your router have a default gw set and can it traceroute to the internet by IP address?
Can your PC traceroute (or ping) the hotspot by IP address?
Can your PC traceroute out to the internet at IP level (not domain name)?
As far as customising the hotspot webpages - simply use an ftp client to connect to the MT box, Download the HTML files in the hotspot directory, edit them to what you like and upload them agian using ftp.
The other way to get the files on and off is by SCP if you happen to have a linux box handy.
Just to confuse things further - I think your ideal solution would be to configure your ADSL modem as a bridge and terminate the PPPoE session on the MT box (username and password on MT box not modem) - That way you don't have a double layer of NAT/Masquerade between your computers and the internet, and you end up with your real world IP address on your MT box. This will make it much easier if you need to open incoming ports to some computers to allow specific services to work.
p.s. feel free to give me call