"Smell this food."
Translation:Cium makanan ini.
Halo teman! I think "cium" is the correct grammar for the imperative form, while "mencium" ("cium" is also used in colloquial speech) is the active verb. Your sentence "Mencium makanan ini." still can have a correct grammar construction, but with a different meaning.
In colloquial speech:
Mencium makanan ini. = Subject + mencium makanan ini.
Subject + smell / smells / smelt this food.
When the subject is dropped, any subject / pronoun can be used depending on the context.
With "makanan ini" as the subject, and stressing the verb:
Mencium makanan ini.
This food smells / smelt. (perhaps, this sentence has no sense or it is incomplete)
I really do not know if the last English construction can be used, and the Indonesian sentence has a meaning with "makanan ini" as the subject. These are just examples of the word order construction in Indonesian.
Edit: After doing a research I can see the verb "mencium" used first in the sentence is part of a subject construction where the translation is "smelling", for example:
Title in Indonesian
Mencium Makanan Kita Bisa Menghasilkan Berat Badan.
Title in English
Smelling our food may make us gain weight.
Selamat belajar! :)