Model doesn't show up in toolbox

Hi all,

I tried to find the most appropriate sub-form for this question, so apologizes if I am mistaken.

Problem: I made a custom model in a custom toolbox. I have tried to attach this toolbox in an email and send it to another user. When this other user opens the toolbox, they say that it is totally empty. We are both using Arc10.1. What is going on?