I ran into a similar issue with my Samsung Stratosphere. Wildman (here) showed me a solution. The issue was that ringtones and notification tones are stored in separate, inaccessible areas in the phone's memory. But apps can also locate them in folders with specific names on the SD card. Basically, you want to copy that particular tone from the ringtone area to a folder on the SD card called "notifications." If Myxer will let you do this, you're good to go. If you can simply get the ringtone on your PC and connect the phone to the PC via USB, you can use Windows Explorer to create the folder on the SD card and copy the ringtone there. The text message app should find it and offer it as a possible notification tone.