So, when loading one of the presaved tutorial flights i.e 'Before Takeoff' or 'Before Top of Decent', for the EHAM-LOWI tutorial, the FMC does not allow any editing. I can copy entries into the scratchpad, I can see all the different pages on the FMC, I just can't actually edit anything.

Any ideas?

I have no idea of what you are tying to "edit" or what you are entering in the scratchpad and what you are trying to achieve.  A little help here in the problem statement please.

Okay, for example, in the tutorial it shows you how to go direct to a waypoint, by copying using the line selector, and then pressing 1L. But it just doesn't place it into 1L.

Another example is trying to offset your planned route. Type the appropriate code into the scratchpad, enter it into the appropriate line, but it just does not transfer from the scratchpad to the line.

Nothing from the scratchpad seems to be able to be input into the FMC. The above are just example, but the premise being that I cannot "edit" any entry within the FMC. Everything stays as it is currently set.

Ah ok, I am going to make a guess here and suspect that because the Tutorial panel files are from five years ago they might not work in current versions. I assume this is Tutorial #2, so what about the first one in the series "PMDG 737NGX Tutorial #2 EHAM-LOWI.FSSAVE" does that allow normal use of the FMC?