I don't believe you can route an external effect via s/pdif with the firepod. Here is the routing suggested from the manual.

"Line Inputs/Preamp Outputs. The FIREPOD features ¼” TRS balanced Send and Return jacks on Channels 1 and 2. These jacks are labeled as Preamp Output (SEND) and Line Input (Return). These connectors will accept either balanced (TRS – Tip Ring Sleeve) ¼” cables or unbalanced TS (Tip Sleeve) ¼” cables. These connectors allow the use of external processors such as reverbs, de-essers, limiters, EQ’s, etc. Simply connect the Preamp Output jack, balanced or unbalanced to the input of the external processor. Then connect the FIREPOD’s Line Input jack to the output of the external processor. The signal is now routed out of the FIREPOD, into the external processor, then back into the FIREPOD. The final, processed signal will be available to record now. The Line Input can also be used as a standard line level input for devices that do not require pre-amplification (such as a CD player, drum machine, sampler and etc.)."

Then if IIRC, and I may not, you have to route the signal from your track to channel 1 if mono or 1 & 2 if stereo. I used to have a firepod but I'm using the FS project now, but I believe that the stereo output bus is automatically routed to these two channels, so if you don't want to or haven't already set up mono channel output busing in cubase you can solo the track in question through the stereo output bus and if your signal is mono just pan the bus to one side and set the recording to the incoming channel (1 or 2.) If you are trying to use a send then this routing will be from the fx channel I believe.

Ok and what I do with the External Effetc thing? How I route this? I'll be able to use the effect only in insert and aux in Cubase?

I'm not sure what you mean when you say " External Effetc thing?" What I have dome with external effects is pass the signal through the effect and record it to a new track, then use the original track (dry) and blend it with the new (wet) track to the desired amount of effect similar to the way I use an FX track. There may be another way to do this but I think you have to record the effect in order to use it. Note this line from the manual:
"The final, processed signal will be available to record now. "

What I want to do is to have my TC M300 (Hardware FX) to act like an actual plugin that I'll insert on my tracks. I don't want the effect in my recording chain I want to add it later to now apply just like a plugin would do. In Vst Connections, in cubase, there a tab where it says External Effect and that is where I want to add my M300 so I can get it to work like a plugin. I tried via SpDIF wich is supposed to work but I wasn't hearing any effects...

Thanks for helping me.

What we are trying to say is is that you may have to solo the channels you want to pass into your effects unit. You would then take the analog output from your audio interface to the effects unit input. You take the output of the effects unit back into 2 inputs into your computer audio interface and record those effects on a stereo or 2 separate tracks. Then you could utilize it in your mix down. There is just no convenient way to monitor this and your entire production at the same time with your current equipment. It's not versatile enough for this. So this is the only sensible workaround. You record the effect on its own track. You'll do this for as many tracks as necessary which he will play back in mix down.

Or is it too cold in Montréal to comprehend this?
Mx. Remy Ann David

Thanks! :) No it's not that cold presently to comprehend! :)

But the M300 manual say that I can use it as an insert via SPDIF!?

But the presonus manual says the only effect loop is on channels one and two so do you think that the reverb units technology will create a new routing for the Presonus? Maybe it will work that way in which case contact the TC for the routing through the Presonus and Cubase.