Good questions. Your instructor/advisor hasn't answered them, or did you ask? They usually don't bite. And if they do, shame on them.
Should the extracellular potential be added to all of these compartments or just nodes of ranvier?
All compartments, unless your conceptual model includes wrapping the myelinated sections in tinfoil and grounding the tinfoil. (don't laugh too hard; one of the strengths of computational modeling is that it allows one to try things that would be impossible or impractical in the real world)
I see from the posts in the forum that for the extracellular stimulation, Vector play class should be used.
Could be used. It's a convenient way to force e_extracellular to follow an arbitrary time course. There are other ways to drive e_extracellular.
My question is that this vector is the electric potential signal in time?
Yes. You'll need a vector that contains the time values, and for each compartment in your model you'll need another vector that contains the local extracellular potential values--unless you know that the normalized time course of extracellular potential follows the same waveform at all points in your model (which it should be if there is only one source of stimulus current, and the extracellular medium is purely resistive), in which case you could use xtra.mod and then just use one pair of vectors to drive its global stimulus current).
I have constant values for each point in space
As expected.
I should multiply it by time?
No. Those values are the values of extracellular potential at specific points in space produced by a constant stimulus current. In the usual experimental protocol, the stimulus current is off except during a time interval on the order of 0.1 to 1 ms. When the stimulus is off, the potential everywhere should be 0. That really should have been explained to you. If you don't have hands-on experience with extracellular stimulation, it would be helpful to read about how it is done (should be able to discover experimental protocols in papers or online), or at least talk with an experimentalist who can describe it to you.
this electric potentials should be added for each compartment separately or can a text file containing all extracellular potentials be loaded once?
How did McIntyre & Grill implement extracellular stimulation? (code available from modeldb.science, not that you should necessarily do what they did). If you want to see another example of how to implement extracelluar stimulation, get
https://www.neuron.yale.edu/ftp/ted/neu ... sphere.zip