First, It was me who gave you support, as it's me who did the plugin. We offer support for bugs and errors, but we can not offer customization work for free.
If you don't know the very basic of PHP, your best shot is to hire a professional to do the changes for you, less time waste.
You could put the code wherever you want it to appear,, be it user-dashboard.php if or any other file in your theme.
$wallet = ModelPaymentPro::newInstance()->getWallet(osc_logged_user_id());
if(isset($wallet['i_amount']) && $wallet['i_amount']>0) {
$credit_msg = sprintf(__('Your current credit is %s', 'payment_pro'), osc_format_price($wallet['i_amount'], osc_get_preference('currency', 'payment_pro')));
} else {
$credit_msg = __('Your wallet is empty. Buy some credits.', 'payment_pro');
}
echo $credit_msg;