The Maltego client's log file can be found in the following directories depending on which operating system you are using:
Windows:
C:\Users\<username>\AppData\Roaming\.maltego\v<maltego_version>\var\log\
OS X:
/Users/< user >/Library/Application Support/maltego/v<maltego_version>/var/log
Linux:
~/.maltego/v<maltego_version>/var/log/
Within the Maltego Desktop Client
Aletrnatively you can go to Application Menu > Tools > Open Logs Folder.
