Once you have the file that you would like touch set as a custom alert, you will need to save it to your SD Card in your phone.
The easiest way to do this is:
Connect your device to your PC via the Data Cable.
On your device, touch the Notification bar to open it and then touch the USB Icon “Select to change USB connection type”.
Select Disk Drive and touch Done.
On your PC, go to My Computer and your device will be listed as a removable disk drive.
Open the disk drive by double clicking on it.
Once open, you should see the contents of your SD Card.
If you do not have a folder named Notifications, right Click and create one.
Once you have created the Notifications folder, open it and drag and drop the files you want to be available for custom alerts into the folder.
Once you are done, disconnect from your PC and your files will be available for selection under Sounds & Display on your device.