You do have a USB port and should not have to use the PC Card Interface.
Not all external CD Drives are bootable, but I'd try to boot with the USB connection as I think it's unlikely that the bios in your machine supports the PC Card anyway.
The USB ports are under the flap at the left edge of the screen at the opposite end from the Compaq Logo. Since you use a docking station you may never have had occasion to open it.
The USB connections are the two holes closest to the centre of the unit. There are two, and it doesn't matter which you use.
So, turn off the machine, connect the USB connectors, and fire it up. It sounds like you know what to do from that point.
I hope you can return the DLink PCMCIA to USB adapter.