Usually the phrasal verb look up is used as follows. These example sentences are from the Oxford Advanced Learner's Dictionary.
Can you look up the opening times on the website?
I looked it up in the dictionary.
The objects of the phrasal verb in those above sentences are what are actually needed to be found.
Suddenly, I saw this sentence from an English learning material which is not popular and this sentence was there alone so there is no context I can add here.
Look up the dictionary if you have trouble translating.
As the dictionary in that sentence is not something searched for but something used as an instrumentality to search for something else, I somehow feels the sentence below is more correct.
Look up in the dictionary if you have trouble translating.
I would like to know what is right and wrong. Thank you.