I figure you have gotten this solved since you have not bumped this question since you asked it. I don't have a Mac but I did encounter the same problem a couple of days ago when installing Windows 7 in Virtualbox on a Debian machine. I thought I would put it here for others to see and try if they too encounter it.
There are two possible reasons for this happening if one is to believe what Google tells you. One reason might be that when installing Windows 7 the installer loose the information about where to find the files because the USB is remounted or something similar to this. The second possible reason can be that your installing the operating system from a USB3 port which seems to cause troubles.
In case one the solution seems to be to push cancel when being asked to put in the CD and the replug the USB flash disk into another USB socket. This forces the installer to remount the disk and should now be able to recognize where to find the files it is looking for. (This was my case).
In the second case it seems that it is enough to swap from a USB3 plug to a USB2 or lower type plug and all should be OK.
Now at least you have something you can try out if it happens again.