Have you checked the Forms Properties? The top left hand of the Form and Right Click.
Check that Additions are allowed. Play with some settings and see what effect it has. Use a copy Database or remember where they were first.
If it is just the one Control on your form that won't allow you to Edit then in Design View, Right click on that Control (field) and check if it is Locked. Look under the Data Tab.
Is all this information in a single table or do you have multiple tables you are trying to add information to with one query as the form's recordsource?
I was going to ask the same question as Bob. Assuming that there are more than one table in your source query, check the join between the tables. (right click on the join)