Bobbie,
The plugin is causing the Order Details page to not fully load. It only displays up to the point where the tax info is added after the billing details. I was able to fix it by changing the line 62 in the simple-tax-exemption.php file from:
echo '<p><strong>'.__('Tax Exempt ID').':</strong> ' . $order->order_custom_fields['Tax Exempt ID'][0] . '</p>';
to:
echo '<p><strong>' . __('Tax Exempt ID') . ':</strong> ' . get_post_meta( $order->id, 'Tax Exempt ID', true ) . '</p>';
Ellis Road Press