Q: how do I open the wireless capture file .wcap created using Wireless Diagnostics using iMac X Yosemite?

Hey Billo68,

Thanks for the question. I understand that you wish to open and view a .wcap file. The following resource provides a great recommendation:

About Wireless Diagnostics - Apple Support

http://support.apple.com/en-us/HT202663

A file ending in .wcap will be sent to the desktop.

You can use an application such as Wireshark to view the capture.

Thanks,

Matt M.