A couple different ways to handle this.
cmbTaxClassID.SelectedValue = tsiNVL(rsVoucher!AcTr_TCLsId)
-- or
cmbTaxClassID.SelectedText = tsiNVL(rsVoucher!AcTr_TCLsId)
You could also loop through each item in the control, compare the text to your value and set that item as selected if found.