Setting the 900AP+ up for repeating mode is easy in concept, but there are a few things that you can do (or not do) to make things go more smoothly than they did for me!
Download the 2.20 firmware from here, but skip downloading the AirPlus Manager Windows application. Although I managed to use it to find the 900AP+, connect to it, and update the firmware, I couldn't get it to successfully change any settings (it doesn't seem to use the same password as the built-in browser-based admin function), and it didn't have the repeater mode setting either. So as much as it's a pain going through the hassle of setting a client computer to connect to the 900AP+'s default IP address of 192.168.0.50, at least once you do it, you'll be able to do everything you need to do via the built-in browser-based admin interface.
Connect the 900AP+ via Ethernet for the firmware upgrade and configuration. You don't want to risk getting a data corruption during the firmware update process, and you'll lose wireless connection with the AP as soon as you set it to Repeater mode anyway.
D-Link has posted some simple instructions here for getting into Repeater mode. Note that you'll also need to set the SSID, WEP mode, and WEP key (if you've enabled WEP) to match that of the main AP. Don't worry about the Channel setting, since that will be overridden once you enable Repeater mode.
Tip: You'll need to find the MAC address of the Access Point or wireless router to which you want the 900AP+ to connect. You can sometimes find this printed on the bottom of the AP or router, but you should always be able to find it in your product's admin screens or utility, usually in the Status section. The MAC address is a 12 character code that looks something like 0090DA4B9CFF (sometimes broken into two character groups like 00-90-DA-4B-9C-FF).
Do yourself a favor and manually (statically) set the 900AP+'s IP address information, then write it on a label and stick it on the AP. Choose an unused IP address in the same subnet, and choose one that will not conflict with the range of addresses handed out by your LAN's DHCP server (if you use one). You may need to check or adjust the DHCP server to narrow the range of addresses it assigns. Since DHCP servers usually start low and go high, I usually assign static IP addresses starting high and working down.
Example: If your DHCP server is set to assign addresses from 192.168.1.1 to 192.168.1.50, assign address 192.168.1.254 (subnet 255.255.255.0) to your 900AP+. The gateway would be 192.168.1.1. I was able to leave the DNS blank, since I didn't enable the 900AP+'s built-in DHCP server.
That about does it for the setup. Let's see if it's working...
ref.
http://wireless.com.pt/forum/index.php?topic=239.0 
