Analog line-out from the DR2d's headphone jack? Make sure the headphone output level is up and remember that you only the mic-input shows up at the headphone jack when recording 4 channel, not the line-input. You can switch between the two to check input levels on the display while recording, but the output does not switch. I can't remember if there is a monitor out on/off switch on the menu which needs to be switched on like there is on the R-09.
But when you say "the computer does not recognize the tascam" it sounds like you are talking about a USB connection not an audio line-out connection. The DR2d does not function as an audio interface. The USB connection is only for transfering previousy recorded files. If file transfer using a USB cable is the problem, remove the SD card and use a card reader.