What step are you at?
Are you at the part or assembly level?
What command are you in?
If you are at the part level and not in a command you probably won't be able to select a part.
Making sure the part is active is important too.
I keep getting to the first step that says "Select a part" in big bold text at the top. In tutorial Modelling a Cover Associatively.
I am supposed to be selecting a part that contains features that will help locate the base planes. It just says to select the base part, and then select a face is the next step, but I can't do that either.
The part is activated. I am in the part editing environment. I have not selected any command because the tutorial (unless I've missed it 5 times now) doesn't say that I have to select any command. It seems to indicate that as soon as you create the part, it dumps you into part editor and it's ready to select the part.
I've played with previous versions, probably have hundred of hours of self-learning wrapped up in V15 (if I recall correctly).
In ~V15, there was an "Insert Interpart Copy" command that allowed the user to pull points and planes and such from another part in an assembly into the part you are editing. I used it all the time, probably in ways it was not intended to be used, but it worked for what I was doing.
I'm just trying to figure out a similar method in ST4.
If you are following the tutorial you should be at the assembly level when you select the part. You are creating a new part in the assembly. You fill out the information in the "Create Part In-Place" dialog and then you click on the "Create and Edit" button and the dialog closes. Then you need to follow the prompts and pick the existing part in order to position the new part. Once the new part has been created and positioned you will automatically be placed in that part (that is the "edit" part of the "Create and Edit"). You can then continue with the tutorial on the "Transition to ordered feature creation" page.
I hope that helps. Let me know if you need more clarification.