There's two ways, although one way may not work (i haven't played with cubase in years so don't know if the second thing i'm gonna say exists)
The first way, and the way i do it most of the time, is to choose the track you want to use the outboard on in cubase, route it's output to a different output on the tascam from the main outputs (i.e output 3-4), pan the track hard left in cubase (to only go out on channel 3), plug output 3 from the tascam to the input of the DOD, connect the output of the DOD to a line in on the tascam, create a new track in cubase with the input being whatever line in you plugged the DOD in to, arm to record to monitor the incoming sound (this may differ from DAW to DAW) and then hit play and EQ away. When you're happy with the results, you need to record the track back in to cubase, baring in mind that the recorded track will be out of time slightly so just drag it back into place once it's recorded.
The second way requires something similar to
Logic's IO plugin (which probably exists somewhere as a VST although it you may have to google it find one). This is basically a plugin that does everything in the step above, bar the physical pluging in of cables, but just in one plugin and doesn't require creating a new track to record the EQ'd sound back in to. However, to record the results you need to either bounce the track in real time, or if you want to use the outboard EQ on another track then you still have to go back through the steps above.
The DOD
may not sound as good as the stock EQ plugins in cubase
but the big thing i've found with using outboard EQ's is that ,without having a graphical representation of what the EQ is doing that so many stock EQ plugins have, you really have to use your ears
