Alright, I will write the actual code for you, its not very difficult really. My way to do this is to create a PrivateMessage object and call the post() method to send and save it, which may be harder to understand for you than to write an INSERT query. But since you dont even know how to write an INSERT query, it doesnt matter, just copy/paste the code and follow the instructions.
This is the complete code:
Code:
$message = new PrivateMessage;
$message->setrecipient($mysidia->input->post("owner"));
$message->setmessage("You have received an item from Admin", "Dear {$mysidia->input->post("owner")}, you have received {$mysidia->input->post("quantity")} {$mysidia->input->post("itemname")} on Atrocity.com. Enjoy!");
$message->post();
All you need to do is to add it below the if...else statement code I showed you in an earlier post. Search for 'public function add()' in file /admincp/inventory.php, and you should find it around line 29-37.