First of all, I'm fairly new to Mathematica and the Wolfram Environment in general. If this question has already been asked I'm sorry for the duplicate, but I haven't been able to track down the issue with searches on this site and the web.
I downloaded the AddOn-Package 'Theorema' and in the installation instructions it says that the package can be moved "into one of the locations for standard add-on packages'. In the AddOn folder that comes with the Mathematica installation there are three subdirectories: Applications, ExtraPackages and Packages. Not knowing where to put the Theorema package, I just pasted it into the Packages folder as stated in the installation instruction.
The problem now is, when I open a given .nb notebook file it immediately gives me an error that
the stylesheet TheoremaCore cannot be found by the Wolfram System
even though I have checked and it exists within the Theorema folder.
The second problem is the 'execution' of the Theorema package. The installation instructions say:
...execute the first command in the notebook
Needs["Theorema"]by placing the cursor into that cell and hitting Shift+Enter.
However nothing happens when I do that. I also don't know exactly what is meant by "cell".
I hope someone has some kind of advice, thank you in advance.