This depends on the Operating System of the App-V Client:
• On a Windows XP/ Server 2003 machine you’ll find the sftlog.txt (yes .txt not .log) in the following directory:
C:\Documents and Settings\All Users\Application Data\Microsoft\Application Virtualization Client
• On a Windows Vista and later / and Windows Server 2008 and later machine you’ll find it in the following directory:
C:\ProgramData\Microsoft\Application Virtualization Client