Can we add anything to the base class PIMEntity?

Yes, feel free to add whatever you want, just don't remove anything that is there. The basic idea is to deal with inheritence, feel free to get as fancy as you want as long as it is easy to verify that you are using the inheritence specified in the project description.

Do we need to create each class in a separate file (or can everything be in one file)?

We would prefer that you put each class in it's own .java file (just so you get used to the idea), but this is not a requirement. As long as PIMManager has a public main() we can run we will be able to test your project (whether everything is in one file or not).

If I try to use object serialization to save/load, the Priority string doesn't seem to work right - how can I fix this?

You need to have the base class PIMEntity implement the Serializable interface (not just the derived classes). Feel free to change the declaration of PIMEntity to include "implements Serializable".

when we load from a file, should we replace our current list of entities or add to it?

Replace

Do we need to implement fromString() if we do not use it anywhere in our code?

No, just make sure it compiles