Apple's "extend" feature uses proprietary settings that are designed to only work with other Apple routers.
So, it would be extremely unlikely that a D-Link router.....or any other router, for that matter.....would be compatible with an Apple product if you are trying to "extend" using wireless only.
All you can do is configure the D-Link device according to the D-Link manual to see what happens. But, unfortunately your chances of making this work are quite close to zero.
The odds improve greatly if you can establish a permanent wired Ethernet connection between the Time Capsule and D-Link router.
That should work, assuming that the Express would be located where it can receive a strong signal to be able to 'extend" it.
In general, a good location for an "extending" device using wireless only, is a point that is about half way between the Time Capsule and the general area that needs more wireless coverage.
Bottom line....make sure that you understand the store's return policy, because you cannot know in advance whether this will work to your satisfaction.
Bob's advice is spot on, but it may not be apparent that the bandwidth (speed) you'll achieve with the extended range configuration will be significantly reduced using a wireless-only architecture. If you were to add an Airport Express (no wires) and put it in between the Time Capsule and the area in which you need more coverage, the Express will be working as a repeater. First, it receives a wireless packet from the TC. Then it retransmits it to your Macs or other wireless clients that are more distant from the TC. And vice versa for packets coming from your distant Mac. The TC will receive each packet, and retransmit it to the TC. The repeating function cuts the throughput in half, more or less.
A much better way to extend the range of the Time Capsule is to run an ethernet cable from the TC to the area where you need better coverage, and connect it to another Access Point (D-Link, Linksys, Netgear, etc). Since no repeating function is performed, you'll get full WiFi throughput in the extended region (whatever is the speed of the added AP). Of course, running a cable may not be possible given your physical environment.
I misspoke (mistyped) in my post above.
It should have said:
"The Airport Express will receive each packet, and retransmit it to the Time Capsule."
When used as a wireless repeater, the AE (or any WiFi repeater) sits in the middle of the wireless connection, receiving the packets transmitted from the base Access Point and retransmitting them on to the WiFi clients that are associated with the repeater (i.e. those in the extended coverage area). And vice versa for packets coming from the remote clients and sending them to the central Time Capsule. That results in significantly reduced throughput.
Another thing you might try is to connect another AP (your D-link, or another brand) to the Time Capsule with the longest ethernet cable you can manage: 20 feet, 50 feet, etc, and position the second AP closer to the area you wish to cover. WiFi propagation can be sensitive to the location of the unit, due to absorption or reflection of the radio waves. Sometimes, moving an AP a few feet one way or another can have a significant impact on the radiation pattern, depending on the environment (wood, steel, concrete, etc). You might get lucky and obtain better coverage from the second AP. Make sure the second AP is operating on a different WiFi channel from the Time Capsule.
Some APs have external antenna connectors, and can be enhanced with more 'powerful' antennae. In fact, they are not more "powerful" but are more directional and focus more of the RF energy in a horizontal direction.