how to calculate the output value of an lm335 sensor
Without a schematic, it is difficult to surmise what might be the problem. 

At a guess, the device is not connected as shown in Figure 4 or 5 of the datasheet, fed from the circuit's Vcc through a an appropriate resistor (R1 in those figures), as described in the datasheet.

R1 is calculated such that current through the device is 1 mA (or up to 5 mA) with a voltage drop across the device for your ambient temperature, much like the calculation of current limiting resistors for LEDs.

The voltage drop calculation is provided on Page 8 of the datasheet.