First is your WiFi Access Point (AP) dlink the only dlink listed in the pull down listing? If you live in a densely populated area your AP's name is not unique enough. You should alter it like Fidos'_Den, Cats'_Lair, The_Fish_Tank, or what you can think of that does not directly ID you (basic security).
If you can't even see the AP list or can't see your AP name at all your WiFi network is not working, no listing in your menu your MacBook's AirPort card is bad. Or, your WiFi Router's WiFi circuit is bad if you don't see its name but you see other AP's (when your physically near it). If you don't see it at one location Vs another then you'll need to move the antenna or the router closer to that direction (more central in your house). Try resetting the Router and think about putting it on a surge suppressor or better yet a small UPS.
OK, you've changed the name and sometimes you can see it, other times not, so we know your system and the router are working. What's causing your WiFi to be unstable? Again, other people! in this case your AP's frequency has too much noise of other AP's around it. You'll need to change out the channel its using or better yet move to the 5GHz frequency (802.11n). You'll need to upgrade your AirCard and may need to upgrade your Router as well it you go to 5GHz (802.11n).
Lastly, you can use a monitoring apps to see what AP's are running around you and there channel/frequencies. Here's one on the App Store I use WiFi Explorer.