What is correct to say: "I don't like it when ..." OR "I don't like when ..."?

I think "I don't like it when" sounds more normal-ish, but it doesn't seem wrong either without the "it". :) I hope i helped.

I don't like it when people can't understand me.

In this sentence "IT" is being used as an empty or 'dummy' subject. It is understood that 'it' relates to people not understanding what you are saying or doing but you are not specifying exactly what 'it' is..

It would be better phrased as: "I don't like being misunderstood."

I don't like it when it rains.

In this sentence you are using 'it' as both an empty subject and an object pronoun. Again, this might be better phrased as: "I don't like the rain..."

"when it comes to" is an interesting phrasal verb, meaning to reach a particular state or point, especially one that is bad or unpleasant. So:

I don't like it when it comes to arguing.

Is probably more correct than:

I don't like when it comes to arguing.

But I am sure you will hear both. Again, 'I' am the object and 'it' is being used as an empty object. However, I think this is better phrased as:

I don't like arguing.