Using a snorkel for swim drills is a very valuable skill to have in your tool belt.  Unfortunately, people get excited when they see they've been assigned snorkel use, not realizing that it's a LEARNED SKILL -- then they get frustrated and give up on it when they get water up their nose.

Using a snorkel will not be easy right out of the gate. It's typically a 3-8 week process to start to get comfortable using it (or 10-12 swim workouts), although it takes even longer for some people.

To begin getting comfortable, the most important cue is having a sensation of "clamping off" your nose internally WHILE breathing through the snorkel.  This is the same thing you do when you go underwater while blowing bubbles; you already know how to clamp your nose off in that scenario, now you have to get good at it with the rubber mouthpiece in your mouth.  

Also, every 3rd breath (or every 2nd breath if needed), try to breath out through your nose to clear out any water that has gotten in there.

Don't give up on using the snorkel!  You will eventually get good at it, and it will become a very important part of your swim practice.