Thanks for your message.
Do you use the latest version? The "Appendix" should be on the left side of the help window if you click Help menu -> Help (or press F1).
The complete help is available online at http://www.hdsentinel.com/help/en , search the bottom left area for Appendix (and Temperature Calibration under that).
If you prefer to check the temperature with an independent device, you may use an "infrared thermometer". They're usually available in bigger stores, sometimes in pharmacies, just make sure the operating range should be between at least 0-80 Celsius (some of such thermometers are used to check fever and works in a small temperature range around human body temperature).
An other (easier) way is to just use a room / office thermometer:
Note: if the calibration is not possible (the computer chassis cannot be opened), an estimated offset value can be determined by checking the first displayed temperature immediately after starting the computer and comparing the value with the environment (room, office) temperature. At this time, the CPU, video card or other components are not too warm and they do not affect the temperature of the hard disk. Of course this is true only if the computer had enough time to cool down to the environment temperature (it was not powered off for at least 8 hours).
(this is described as-is in the Appendix -> Temperature calibration field).