Set up the Terms for each account by going to the Setup menu and selecting Account Preferences (see the invoice section).

Select Default as the Account.

Highlight Invoice in the left list, you will notice that the Bill Terms is the first item in the right list.  Change the setting to whatever you would like the default Terms for all accounts to be set to.  It should include the text you want printed, so for example: Terms:  Net 30. 

Next, select different accounts in the accounts drop down and set their terms individually (if they will be different than the default (ie Net 30)).

After that setup is done, the terms will be set automatically when you run a bill.

To override terms for a specific bill, bring up the bill on the Search Bills screen by either clicking view on the Create Warehouse Receipt Screen or going to the Search Menu and selecting Bills.  The Search Bills screen will appear and the terms can be edited their.