I can't define my function in calculator

Try this. I reorganised the code a little (you should check this, because I didn't really have anything to check it). As you can see, I broke down the code to make the calculations simpler. You need to load this code (put it in a file and use load("filename.il") in the CIW or in your .cdsinit file), and then call MYthreshVoltageReg() to register it. There are quite a lot of solutions on Cadence Online Support for defining calculator functions. Note that in IC615 you also have the "Expression Editor" which allows you to define dependent expressions which would be an alternative to writing this code - but it depends on how often you're likely to use it, I guess.

procedure(MYthreshVoltage(current)
let((di diMax xMax vto)
  di=deriv(current)
  diMax=ymax(di)
  ; this can probably be done with xMax=xmax(di)
  ;xMax=cross(di diMax 1 "either" nil nil)
  xMax=xmax(di)
  ; last value in function is return value
  vto=(ymax(di)*xMax-value(di xMax))/diMax
  )
)

procedure(MYthreshVoltageCB()
  calSpecialFunctionInput('MYthreshVoltage nil)
)

procedure(MYthreshVoltageReg()
  calRegisterSpecialFunction(list("MYthreshVoltage" 'MYthreshVoltageCB))
)

; call MYthreshVoltageReg() to register the function as a User Defined SKILL
; Function in the calculator