Instructions
Use the "E-mail Support" on the official Microsoft Xbox website to write to Microsoft and request a replacement disc for your wireless adapter. Inform the technical support representative that your wireless adapter did not come with a disc or you misplaced the disc since your original purchase (whichever is applicable) to request a replacement.
Ask if you can purchase just the install disc for the wireless adapter from your local GameStop. GameStop has Xbox 360 consoles on display, which means that they have copies of the install disc somewhere in the store to be used for these demonstration units. If the store has more than one disc available, you may be able to purchase it.
Return your wireless adapter to the store where you purchased it and indicate that it did not come with the required disc (if applicable). If you just purchased your wireless adapter and got it home only to find out you don't have that all-important disc, return it to the store and exchange your unit for an unopened one, which should have the disc inside the packaging.