First, you'll need a level converter circuit. There are many on the network. You can find a good write up on them and links from adding a serial port to a NSLU2. I always use the Parallax USB2SER that I bought years ago. It is very reliable, and I've had several provide years of good service to me. They convert 3.3V directly into a USB serial port, which vastly simplifies power and cabling. Since that's what I used for this project, all the instructions are geared towards it.
First, you'll need to disassemble the D-LINK DIR-615. There's only two screws so this is easy. Place the D-Link with the label with the serial number face up. There are four rubber feet on the bottom of this unit. Carefully pry up the two nearest to the Ethernet ports. Remove the screws found under them. The screws are clearly visible in this photo.
Carefully remove the lid. It should pry off just above the plastic bar that runs over the ethernet ports. You'll see a green PC board. On the right hand side of the unit, you'll find a header with 4 pins labeled CON5. They are labeled VCC, TX, RX, GND. This is the serial port. In my unit, it was hidden under the internal antenna, so the photos show it moved over a bit. Note also in figure two the board says "DIR635B1" on it...
I connected these pins to a ribbon cable and ran them out to a header that my USB2SER could fit on. I connected them so that they would match up to my USB2SER which had pins in the order GND, RX, TX, RES. The last one is unconnected. TX should go to RX, RX to TX and GND to GND. I didn't connect VCC, since it wasn't needed. USB2SER gets its power from the USB port. I reconnected everything and plugged the USB2SER into my computer. I threaded the ribbon cable out through the holes in the plastic, which is what you see in the first photo. Here's everything hooked up. I didn't include a photo of the USB2SER plugged into the serial port of my laptop for obvious reasons...
I see that it has a uboot bootloader:
% tip ucom0
U-Boot 1.1.1 (Jan 19 2007 - 11:08:07)
CAMEO uBoot Linux Loader version: 22.214.171.124
DRAM CS base 0x00000000 size 32MB
DRAM Total size 32MB
before entry mvFlashInit
Flash: flashStructGet manu 0x89 id 0x17
INTEL 28F640J3A (64 Mbit)
Size: 8 MB,Bus Width: 1, device Width: 1.
Flash base: 0xff800000,Number of Sectors: 64 Type: REGULAR.
[8192kB@ff800000] Flash: 8 MB
Addresses 20M - 0M are saved for the U-Boot usage.
Mem malloc Initialization (20M - 16M): Done
*** Warning - bad CRC, using default environment
Soc: 88F5181 B1
CPU: ARM926 (Rev 0) running @ 500Mhz
SysClock = 166Mhz , TClock = 166Mhz
USB 0: host mode
PCI 0: PCI Express Root Complex Interface
PCI 1: Conventional PCI, speed = 33000000
Net: egiga0 [PRIME]
Hit any key to stop autoboot: 0
So that's how you add a serial port.