I had the same problem with WireTap Studio not working with Yosemite, and when I contacted Ambrosia software I was told that the product had reached the end of its shelf life and would not be updated. Pretty cheesy since I bought the software only one year ago for $69. They offered a solution which did not work. I was able to find a way to get it to work, so if you're stuck and have tried other things, here's what worked for me:
1. I removed the AmbrosiaAudioSupport.kext file from the System/Library/Extensions folder and reinstalled it directly into the System library: System/Library/AmbrosiaAudioSupport.kext.
2. I deleted all the Ambrosia preference files from the User Library:
User/Library/Preferences/com.ambrosiasw.exporter.plis
User/Library/Preferences/com.ambrosiasw.wiretap_studio.au_cache.plist
User/Library/Preferences/com.ambrosiasw.wiretap_studio.LSSharedFileList.plist
User/Library/Preferences/com.ambrosiasw.wiretap_studio.plistfrom
Upon startup WT Studio created new preference files, and was now able to recognize and use the AmbrosiaAudioSupport.kext file and record directly from the internal Apple Audio.