Try this.
Put a text box on your main form. In the CONTROL SOURCE of that put this:
=DSum("[amount owing]","levies","[MembershipNum] = " & [txtmembershipnumber])
Try this.
Put a text box on your main form. In the CONTROL SOURCE of that put this:
=DSum("[amount owing]","levies","[MembershipNum] = " & [txtmembershipnumber])