Default samping rate is 11025 Hz
Default frame size is the highest power of 2 which is less than 0.03 * sampling rate. For default samping rate the frame size is 256 samples. You can use this formula for calculation:
pow2(floor(log2(0.03*fs)))
Default overlap is 50%.
So the default frame increment is 128 samples. To get the offset you need to multiply the frame number on frame shift (128) and divide by sample rate (11025).
You can find the details in the header here
http://www.ee.ic.ac.uk/hp/staff/dmb/voicebox/doc/voicebox/melcepst.html