Learn Russian: Господин Иванов ждёт вас в кабинете. - Mr Ivanov is waiting for you in the office.

It is very similar! Like those words (such as гражданин becoming граждане), господин drops the -ин suffix in the plural. However, it takes an irregular ending: the plural is господа (gentlemen or ladies and gentlemen).

We say 'waiting for you' in English. Why isn't there a preposition like для before вас?

The Russian verb ждать (to wait) directly takes an object without a preposition. When you are waiting for a specific, animate person, that object goes in the Accusative case. Here, вас is the Accusative form of вы.

Does в кабинете mean he is waiting in a cabinet or cupboard?

No, кабинет is a false friend. In Russian, it means a private office, a study, or a professional consulting room (like a doctor's office). If you want to talk about a piece of furniture for storage, the word is шкаф.

Is it common to use Господин and Госпожа in everyday Russian?

Not really. In modern Russian, these terms are highly formal. You will hear them in business, diplomacy, or news broadcasts. In normal polite interactions, Russians prefer to address someone by their first name and patronymic instead of using a title and last name.