ckier,
In Filemaker, a checkbox is is really just a regular field formatted to show a checkmark next to selected values. So for instance, if your field contains the values "Yes" and "No", to script the checking of the box what you need to do is enter the value of either "Yes" or "No" into that field. That is typically done using SET FIELD:
Set Field [TableName::FieldName; "Yes"]
This will checkmark the "Yes" value. To remove all checkmarks you simply clear the value from the field:
Set Field [TableName::FieldName; ""]
You should be aware that if you entered the text "Maybe" into the field, neither YES or NO would be checked nor would you be able to see the value MAYBE. The value list that you have assigned to that field must contain all valid values that you want to display. In this case you would need to have YES, NO, and MAYBE as values.
-Striker