The Freq() command takes two arguments: wavelength and time. TBO_Yeong and linuxgeek96, I'm not sure where you got your wavelength numbers from, but neither of them appear to be correct. The approximate value that I got was 487, using this formula for the wavelength argument with middle C's frequency, 261.63Hz:
Wavelength argument = (6000000/frequency - 62)/47
Now, to deal with the time argument. The formula for this is a little more complex, since it doesn't have an explicit form, only implicit:
Desired duration = (47*time argument + 48*floor(time argument/wavelength argument) + 71)/6000000
However, if we give floor(time argument/wavelength argument) an approximate constant value instead (I'll give it about 50), we can get an explicit form:
Time argument = (6000000/desired duration - 2500)/47
If we wanted a note length of 1/4 of a second, this would give us a time argument of approximately 31900.
Putting it all together, to play middle C for about 1/4 of a second, you would use the following code: Freq(487,31900).