Hi again Andrew,
First of all, thank you very for your help.

Here I attach a file containing the design matrix that I have constructed for modeling only two time points. Could you please tell me if it is correct? MSL and CTL are the two taks, "ses" designates the corresponding session/time point, and "su" designates the corresponding subject.

I'm using the following constrasts:
-to test for task: [1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
-to test for session/time point: [0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
-to test the interaction between task and session: [0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]

Finally, given that it is a within-subjects design, I have set the exchange blocks as follows (remember that I have 21 subjects):
[1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21]  Is this correct?

By the other hand, could you please show me in with a little example how the design matrix should look like for the case when modeling the three time points? I would really appreciate it.

Again, thank you so much in advance.