This should be self-explanatory, but just in case it's not: your F-ratio value goes in the F-ratio value box, you stick your degrees of freedom for the numerator (between-treatments) in the DF - numerator box, your degrees of freedom for the denominator (within-treatments) in the DF - denominator box, select your significance level, then press the "Calculate" button.
If you need to derive an f-ratio value from raw data, you can find an ANOVA calculator here.
Significance Level: .01.05.10
Enter your F-ratio values and degrees of freedom for the numerator and denominator, and then press the button.