You asked about the difference between “in” and “at” when referring to a location and you asked about their uses in the following two examples:
1. The meeting will be held “in” the conference room “at” our Southmark office (the name of an office building).
2. The equipment will be installed “in” the canteen “at” ABC Hotel “in” Hong Kong.
Both of your examples are correct. The rules, briefly, are as follows:
“at” - is used to talk about position at a point, and is common with the proper names of buildings, e.g., at the Hong Kong Bank Tower; is also used for group activities (e.g., at a conference, at a meeting)
“in” - is used for position inside large areas (e.g., in the sea, in Shanghai, in the forest, in the parking lot).
Sometimes either “at” or “in” could be used if you are taking a different perspective on the place, for example:
“The cat's somewhere in the house.” (a large area)
“We're meeting at Susan's house. (a meeting place)
In fact, in your first example,
1. The meeting will be held “in” the conference room “at” our Southmark office.
you could also say “in our Southmark office” (inside a large area), but in this case we're talking about a meeting place (and we also prefer to avoid saying “in” ... “in” in such close proximity.
