Most stock messaging apps wont' allow the selection of custom ringtones. Either:
1) move the tone to where your phone keeps its tones, and it might find it the next time you reboot, or
2) Download and use Handcent for your text messaging. It has a ringtone picker that will find your custom ringtones. If they're in the right places (there appears to be an issue with the HTC One M8)