Arduino Razor IMU with Serial.write(id, 2): Can't have 2 parameters
According to the Arduino.cc page on Serial.Write, it does support 2 arguments, where the first argument should be an array name (or a pointer to an array, which is the same thing) and the second argument should contain number of items you wish to transmit over the serial link. Line 535 of the Razor_AHRS.pde file you linked to does conform to that specification and should not cause a compile error.

I installed the latest Arduino IDE (1.0.2) and after uncommenting a single line to select hardware the code that you linked to compiled successfully. Looking at the Arduino Changes website it looks like Serial.write(buf,len) was introduced in version 0016.

Please verify that you have a new(ly) version of the Arduino IDE and then update your original post with the error message you are getting. (Altough the Arduino IDE has, at least historically, been rather bad at giving descriptive error messages, in my opionin.)