Does your table have an auto-incrementing ID field? If not, you will need to manually specify the value for the ID in your INSERT statement.
You can check if the ID field is auto-incrementing by using the Object Explorer, navigating to the table and expanding the Columns node. Find the ID column, right-click on it and select Properties. If the Identity property is set to False it means that the ID field is NOT auto-incrementing.
Your other option for adding a row to the table is to navigate to the table in Object Explorer, right clicking on it and selecting Open Table. You can then go to the last row in the grid and manually enter the values for the columns.
INSERT INTO [Table].... If not, please post the real code.