Davo is correct that some basic port forwarding knowledge certainly helps getting this set up.
Port Forwarding doesn't need to be intimidating, fortunately. The concept is pretty simple.
Think of a Port as a doorway, and most firewalls lock all the doors for security. UPnP is a way to make the firewall open the door if someone knocks.
You could also manually unlock the door. This is basic port forwarding. You have to tell the router what computer on your network the door goes to.
With WebAccess, you want that door (port 9000 by default) to open up into your NAS (and your files).
So your router, which has to physically allow data into your network, has to be TOLD that it's ok to let port 9000 be open and any traffic through that port needs to go to your NAS. To do this you need two pieces of information. 1) Which port, 2) Where it's going.
All router interfaces are different. Some split out two traffic types (TCP and UDP) and some just let you do both at the same time. WebAccess uses TCP. When it asks you for the IP address, it wants the IP address of your NAS.
If your NAS is getting random IP addresses every time it reboots this can cause issues - there are ways to make it always get the same address, again this is a router function and the interface varies, it's called 'DHCP Reservation' on a lot of routers. You can always tell your NAS to use a specific IP address as well, just make sure it's not one of the addresses that is getting handed out to other machines.
For your router, it looks like its port 9000 to 9000, go ahead and use both, and the ip address is the address of your NAS. This information is in the NAS console if you don't know how to look it up on the router.
Should be that simple.