I have started the 4th map in the campaign in Dungeon Keeper Gold. It introduces the Workshop just as the previous map introduced the Library.
With Library I had no trouble. I built it, Warlocks came through the Portal and I could do the research.
But this doesn't seem to be working with the Workshop. I built a pretty big one and I have all other kinds of rooms in my dungeon as well. But no creatures that would like to work there has been coming through the Portal. Can it be that I am doing something wrong?

Are you building the workshop early enough in the level? As I understand it, there is a limit to the number of creatures you will be able to get through a portal (you'd have to do some research for more details; I'm not clear on those details myself). If you wait too long, you might be hitting the creature limit. You should get a troll in the same level the workshop is introduced, after you build at least a 3x3 workshop.

Sometimes though, you might not get creatures that default to working in the workshop (or default to researching). You can assign creatures to work in a room by picking them up and dropping them into that room. Note that some creatures will NOT do some jobs. For example, warlocks won't work in a workshop, and trolls won't work in a library. Some creatures, like beetles, don't have a default job, but they'll do whatever you assign them to do (they might not do it well, but they'll work at it).

I think Dragons might be the creature that tends to stay busiest (they'll try to research if they can, then try to train, then they just sleep), and Horned Reapers are the fussiest (in my latest play through they wouldn't research or work in the workshop, but assigning it to a guard post room keeps it busy so it doesn't get bored and angry, and a guard post doesn't have 'work fees' like a training room).

One other thing, while I'm thinking about it. In some levels, you may have gold supply issues. The traps and doors that are made in the workshop can be placed and then sold for gold. This gold doesn't appear as a pile in your treasury, but it is added to your gold total. There is a gold pop-up when you sell a trap, but there is no such pop-up when you sell a door (though you do get the gold). The "stronger" traps/doors sell for more, take more time to make, and have a larger minimum workshop size requirement (a 5x5 workshop should be able to make any available trap/door). So it can be a good idea to assign creatures to the workshop if you don't need them doing something else. This can also occupy most of the creatures that train naturally and prevent them from spending your gold by training; alternately, you could just place doors on all openings leading into the training room and lock those doors (assuming you built it such that locked doors can block all access).

I think every creature can work in the workshop, simply pick em up and drop them in in the room, same goes for the library and training room. Every creature has his/her favorite room e.g. bile demons build eat sleep, but if you put them into the training room it will be training/build/eat/sleep (building happens when the training room is full or the bile demon reached max level)

You can drop any creature in any room you own, but that doesn't mean they'll actually work. If you drop a warlock in a workshop, he'll just walk out instead of staying to work. Same if you drop a troll in a library.

Thank you for all the tips and explanation.
As Bookwyrm627 hinted: I hit the creatures limit so new creatures didn't arrive at my dungeon. Once one of existing creatures is gone/death, a new one arrives and that would be a troll in that particular case :)

At the moment, I think I am familiar with game mechanics well enough, but I mostly struggle to make the game working properly. It doesn't run too smooth, and especially when the dungeon becomes bigger (20+ rooms) , the game's responsiveness is pretty bad.
I am tinkering with the settings. I hope I can do something about it, because I really like the game and its dark setting.