you realise if I dumb this down much further I'll have to get out paper and crayons
OK, let's set some constants here:
Your PC's IP: 192.168.1.115
Your ADSL Router's IP: 192.168.1.2
Your AP's IP: 192.168.0.254.
Our goal:
To get the AP onto the 192.168.1.x subnet
So, step by step:
1 ) Change your PC's IP address to 192.168.0.115
To do this, go to Start > Settings > Control Panel > Network Connections
OR
Right-click on My Network Places (on your desktop) and click on Properties. Then Right-Click on your LAN Connection Icon, and then click on Properties.

Then Click Internet Protocol (TCP/IP) and again select Properties

Select Use the following IP address and fill out the fields as required:
In your case, you'll want the IP to be 192.168.0.115, the subnet mask remains 255.255.255.0 and the gateway and dns remains unchanged.
This will put the PC and AP on the same IP range. Disconnect the Router from your PC and connect the AP directly to your PC. Make sure the AP is turned on.
Click on Start > Run. Type "cmd" (without speechmarks) and press enter.
you will get a window that looks similar to DOS. Type in the following:
ping 192.168.0.254
and press enter....
If you can get a response, then move on to step 2. If you get a request timed out, then you may have a problem.
2 ) Use the crappy SNMP to setup the crappy DSE AP crap
3 ) In the configuration (as per the manual) set the AP's IP to 192.168.1.254. The subnet mask remains 255.255.255.0, set the gateway to 192.168.1.2
4 ) Repeat step 1 to bring your PC's IP back to 192.168.1.115 or DHCP
5 ) I dont know if your router has a built in network switch, if it does, hook up your PC and AP directly to that switch
This should have all your devices on the same subnet and talking to one another.