The issue you are running in to is a side effect to the stuff we added in Mockups 3 (namely, the Font Awesome icon library http://fortawesome.github.io/Font-Awesome/) that hasn’t make it to myBalsamiq yet. We are working hard to bring the two into parity, but it’s a large project and we are a small team.
We do have some work arounds, though.
We have a FAQ on how you can try to make your project more myBalsamiq friendly here: http://help.balsamiq.com/mybandb3d but that has a lot of limitations, unfortunately.
A better solution may be to have your clients install Mockups 3 ( https://balsamiq.com/download/ ) and then you can send them the BMPR file directly. They will be able to open files even after the trial expires.
The third (coolest but most complex) workaround is to use the webdemo we have on the homepage (http://webdemo.balsamiq.com/). You can create one specifically for you by adding ?pid=m_bass to the url (IE http://webdemo.balsamiq.com/?pid=m_bass - you can change m_bass to whatever you would like) upload the project to there, and then share it with your customers. The webdemo is built on top of the tech that we added in Mockups 3 (and will eventually make it to myBalsamiq) so there shouldn’t be any compatibility issues. It can even be used for real time collaboration. There is one caveat, however, and that’s the project will self destruct after an hour, so make sure that you budget time accordingly.
Let me know if any of those work for you, and if you have any other questions