How can I insert a null value into my table ?

Since version 2.2.3, you have a checkbox for each field that can be null. Before 2.2.3, you had to enter "null", without the quotes, as the field's value. Since version 2.5.5, you have to use the checkbox to get a real NULL value, so if you enter "NULL" this means you want a literal NULL in the field, and not a NULL value (this works in PHP4).