I checked DLINKs website, this device is ment to be an access point from the get go, so it is much simpler then turning a router into an access point.
There is a switch on the back of your DAP that says access point, put it in that mode.
With your computer connected to the internet (wired or wirelesss does not matter for this step), open a command prompt and type ipconfig. Write down or remember the default gateway address.
Type that gateway address IP into browser and open webgui for your modem router.
Login and find where it says DHCP server address range. You can just make the range be .150-.199 (the router might want the full IP addres so just use the first three sets of numbers of the gatway ip, so if the router is 192.168.10.1 then it would be 192.168.10.150 - 192.168.10.199). Now save and apply the settings.
Disconnect your computer from wired and wireless internet, then go into network and sharing center, change adapter settings, select your ethernet adapter, right click and hit properties, now select IPv4, and select settings. Input 192.168.0.99 for IP, with 255.255.255.0 for subnet and you can leave gateway ip blank as well as dns.
Here is a writeup on how to change it for each windows version: http://www.howtogeek.com/howto/19249/how-to-assign-a-static-ip-address-in-xp-vista-or-windows-7/
Now you can plug the DAP directly to that computer with an ethernet cable.
Login, username and password default should be admin
You should start out in the setup page
Select manual wireless network setup. configure your 2.4 and 5.0 band wifi settings, for testing purposes I would suggest making a different SSID then the one of your primary router. under security select WPA Personal and for WPA mode when it displays the Auto WPA and WPA2 is good, if you do not have any wireless G devices then select N only under mode for the 2.4 band and make CIPHER TYPE be AES, if you have G devices then leave settings on defaults.
Save settings
Select network settings at the left.
Select static ip from the dropdown list, for IP type the first three numbers of your routers gateway ip address, and for the forth number use .2 (so for my example of the router being 192.168.10.1 use the address 192.168.10.2), For the subnet use 255.255.255.0, and for the gateway use the routers gateway ip address.
Save settings.
You can now unplug your computer from the DAP, reboot dap and plug it into router.
Go back and change the adapter IPv4 settings on your computer to Auto and then you should be able to connect to network and internet via the SSID you set on the DAP.
If all is good you can access the config page by typing in the x.x.x.2 ip address you set for the DAPs Ip address, you can then change the SSID to match your routers one (this step is optional, if you dont want them to be called the same or if you are not going to have your motorola router provide wireless this step is not necessary).