The items that Adrian has quoted or referred to are very out of context compared to your original post, so I'll take a whack at it.
I have a DMA2100, and it does work, so take heart. They were a little quirky under Vista, but with 7 they are just fine.
First off, how is it connected to your computer? Wired or wireless? Is your switch/router using MAC address filtering? Is Media Center completely set up on the computer?
Since you can ping the extender, I'm going to assume the connection is okay, so here's what to do:
Start by making sure that the extender isn't installed or active in WMC
1. Turn on the extender, and go to the computer.
2. Open media center, and select add extender from the menu. Follow the instructions
3. During the setup, you will get an access code to put into Windows for the extender.
4. This is important. Make sure that your firewall is not blocking the extender's connection to the computer. Your TV will tell you if the extender cannot talk to the computer. If necessary, disable the firewall temporarily.
5. If you have MAC address filtering enabled on your router, disable it temporarily and copy the extender's MAC address into the router's filter settings field. Don't re-enable the filtering until the extender is working.
6. WMC should configure the firewall during the setup procedure unless you're using a third party FW, in which case, you may need to do it manually.
The biggest obstacle here is usually MAC filtering, or the firewall. The installation should go very smoothly otherwise.
And don't expect a lot of help from Linksys on this one. They stopped support for the DMA2100 last year sometime. At least that's when they stopped answering most people's support requests.