Step-by-step guide
You can do this by using maxvalue() to compute the maximum value, and then find() to discover where it occurs. For example:
g = read("my_data.grib")
m = maxvalue(g[1]) # maxvalue works on all fields given to it; we probably only want one
print(m, find(g[1],m)) # prints a list of lat/lon pairs where the value occurs