That is actually pretty simple. On your form, create a checkbox and textbox. I'll call them Check1 and Text2. nder properties, set the default value for Check1 to false, and set Enabled property for Text2 to disabled. Create an event for AfterUpdate on Check1 as follows:
Private Sub Check12_AfterUpdate()
If Check1.Value = True Then
Text2.Enabled = True
Else: Text2.Enabled = False
End If
End Sub
This will result in the form defaulting to checkbox unchecked and the textbox disabled. When the box is checked, textbox will be enabled. If it is unchecked, the textbox will go back disabled.
Hope this helps.