When you look at the nominal grid you will see a blue bat at the top, this is where you enter details for new lines to be added.
The fields are as follows:
Order - Enter a number here to determine where the line will appear on the grid. It will auto-populate with the next number in sequence
"AC or CS" - This stands for "Account or Cash". Essentially you can choose if this line applied to Cash accounts, non-cash account, or both.
Parameters - this is where you set the filters, this is added after the line has been created
C. - This is the company (from the list in the Companies section of Lookups) that the line applies to. For example if you set this to company "A" then invoices for company "B" will not pick up this line.
Prod Sale Nom Ac etc - these are where you enter the various nominal codes that this line will pick up when posted.
Once you have set the required values click the "Insert" link on the far right to add it to the grid.