A number of points:


1. Did you know you can change the direction of travel the cursor/selected cell goes in after you press Enter from down one cell to right one cell?
2. You can lock cells, then protect the sheet and choose the option of the user not being able to select locked cells, then when the user enters a value and presses Enter, the selection jumps to the next unlocked cell in the direction opted for in point 1 above.
3. If you convert your table into a proper Excel Table (I think it's a List in older versions of Excel), when you add rows at the bottom of the table, the table expands and formulae outside the table adjust to accommodate the whole table.

So you may need very little code indeed.

But, and it's a big but, I'm not sure if Excel 97-2003 version works in the same way (the version of Excel the file you attached seems to have been made by). So before I offer a solution, can you confirm which version of Excel you are using? If it's an older version I'm going to have to use a computer I don't use any more which has this old version on it to confirm what I've said above and work out another solution(s) for you.