I am sorry i can't, you need to run that file on a real DS to rip your firmware so that desmume will sign on to Nintendo WFC with the real DS's "login information"
a real ds that u have to buy
you could try to find a friend with one too, just be sure they do not use Wifi alot. because you wont beable to go on if they are and they cant if you are.
can't i downlaod something to use or something
No, if you find a ds firmware for download, then the chance of many, many other people have downloaded it to, if they are trying to use it in desmume then when ever 1 person is on wifi no one else can, and as soon as they get off, someone else trying to get it working will happen to find them self online and no one else will be able too.
think of it as a big crowd of people waiting at a bus stop, every hour a bus comes but only has room for 1 person. the crowd pushes and shoves and someone makes it on and the bus takes off, leaving everyone else stranded, waiting for the next bus, and each bus that comes only has room for 1 person, repeating the process.