Yep, I'm stuck on this too. Ironically before I needed to find any protestors there were quite a few happening in the Spice District, now I can't find any!
Your own sim can use the various interactions to make other sims join your cause/promote your cause but I couldn't find a way to donate when it was a protest my own sim had started. lol
There probably is a super easy way to do this that I'm missing. :|
*edited: To start a protest @EmPemm just click the ground and choose start protest, make sure your sim has chosen a cause first though.