Put the summary of the bug you're having here
What I expected to happen was...:
Play with a model changing resource pack.
What actually happened was...:
Deselected resource pack
Steps to Reproduce:
1. Start 14w11b
2. Select model changing resource pack
3. Exit resource packs, and go back.
4. It disappeared from the selected list.
Comments

block model of 14w11b use a different format from old snapshot.
be sure you are using the latest format
and this one works
http://i.imgur.com/aAKDJ7f.png

Look at the development console , there are probably error messages about your resource pack.
Jit-Yu Wang would you mind posting a link to that one? I want to see if I can get that one to work.
Could you perhaps include this resource pack to this report?