I downloaded LT3942 Demo Circuit from the following URL.
Then I opened lt3942_dc2404a.asc with LTSPICE XVII and did not find the LT3942 symbol.
Could you send the LTspice model?
Please perform an update on LTspice
You can do it by going to Tools-> Sync Release
after this the model should be in your library
Thank you for your ansewr.
I will update LTspice XVII in my PC.
I close this question.