On my system (Windows 8.1) they're in: "Documents\IK Multimedia\ARC System 2\Measurements".
Sonar Version: X3e Producer Edition x64
Audio Interface: Focusrite Scarlett 2i4, 2.5.1 Driver
Settings: 24-bit, Sample Rate 48 kHz, ASIO Buffer Size 192 samples
Computer: Intel Core i7-3630QM@2.40GHz, 8 GB RAM, Windows 8.1 x64
Soft Synths: Kontakt 5, BFD 2, AAS Modeling Collection, Pianoteq 5, Addictive Keys
Effects: Amplitube 3, Melda Creative Bundle, ARC 2, Melodyne Editor, Breverb 2
MIDI Controllers: Novation Impulse 61