Jim has adequately answered your specific question in comments, but I figured this deserves a complete answer:
A misplaced modifier is when you place a modifier (word, phrase, or clause - typically adjectival) in the wrong position in a sentence. The result is a that it winds up modifying the incorrect term in the sentence.
In your example above:
Weighing 150 lbs is modifying collar, not Golden Retriever.
To write the statement properly you could write:
The Animal Control Representative described the lost puppy as a Golden Retriever weighing 150 lbs. and wearing a silver collar.
The Animal Control Representative described the lost puppy as a 150 lb Golden Retriever wearing a silver collar.
Now the modifier weighing 150 lbs is located in the correct place to modify Golden Retriever. Or in the second example, weighing is unnecessary and has been deleted.
If you wished to change (the verb) weighing to weighs (present continuous tense to present tense) as you had asked about, you would modify the statement slightly.
The Animal Control Representative described the lost puppy as a Golden Retriever that weighs 150 lbs. and wears a silver collar.