What happens exactly when you try to enter something - can you be more specific?

Are you following these steps...

From the Main QIP Master Menu:
1. Select 4. Housekeeping <Enter>
2. Select 4. Tax Tables <Enter>
At this point, do you get a warning message?
3. Press the Down-arrow to select the Tax Group you want to change
4. Press <Enter> on the Description field
5. Select Customer, press <Enter>
6. Now, depending on how your Tax Jurisdictions are setup, you can press <Tab> or <Enter> to move to the PST field.
7. Type in 7.50, press <Enter> to accept the new percentage.
8. Repeat steps 6 & 7 until you have updated all the necessary PST fields, remembering to press <Enter> afterwards to accept the changes
9. Press <Esc> then select 'Save' from the option box to Save your changes to this Tax Group.

If you cannot do this as explained, we will need to know exactly what does or does not occur at any given point above.

Perhaps there is something else preventing you from making any changes here, however, QIP itself has no option to disallow edit of these fields other than what we have already stated: If you have any open invoice batches, or if your Tax Summary has not been printed and cleared, QIP will NOT save the changes you have just made.