The thing is that I can't even change the payload of the rocket. It just doesn't load up the payload menu

Yeah, Bob has it right. I designed the first payload to be significantly overloaded on start. Very KSP like. If you try and alter the rockets payload during the mission setup all you can do is clear it out and then you wasted about 70k of mission weight you cant get back.
Just launch it, and if you wanted something else not in the initial payload, launch a followup supply pod, I gave you 5 for free just for that.

Hey Bob, do you have a save file with a somewhat developed colony and Universities in it?
I need a test save to begin coding and fixing NASA Edu. I know you wanted that sooner, but I have to play the game to make the save that can test against that. That will take me some time.
If you have a save. put it on a fileshare somewhere on the internet and put the link below and I'll grab it.

You still don't get it. I can't do anything about the rocket or the payload. I just won't load up the screen. I don't know what is happening but it only happens when I load your mod

Folks need to abandon all saves every time there is a patch. Hotfix is chancy as well but patches from the dev seem to have significant re-writes of code to the point older code running threads with older references get hosed. Understandable, since the things they are planning and doing are quite dramatic. There will be dramatic code changes.
Case in point. This monday, 3 days from now, will be another hotfix.
I already have it through beta access. There are changes there, that if I didnt go back and redo my mods I literally just redid, then come monday things for me would be broke, again.
Its why my mod updates are slow. I am literally redoing code for things I did already, every 3 days when I get a new code dump.
No complaints from me though. I would rather be in the know early then be surprised on patch day.

Final thoughts, just abandon all save games when large patches come out. Hot fixes should be fine, since the idea is to fix things that were accidental in the patch but can be affected as well.