Let's determine if the problem is in Excel or in NodeXL. Try this, which will create a simple Excel table:
1. Create a blank Excel workbook, NOT a NodeXL workbook.
2. Enter text (any text) in cells A1, B1, A2 and B2.
3. Select cells A1, B1, A2 and B2.
4. In the ribbon, select Insert, Tables, Table.
5. In the Create Table dialog box, check "My table has headers."
6. Click OK. You now have a table.
7. Enter text in cell C1. Does a new column get added to the table?