Hi, I seem to have a problem with some games due to the "frame-rate", for example "Dishonoured" worked well on my computer and i was very happy with the game, however "Dishonoured 2" does not run smoothly at all on my machine due to a disparity between the frame-rate of the game and the frame-rate capacity of my pc (Dell optiplex 790). I do not have funds to upgrade my machine. When I am looking for new games to play why do the creators not specify the frame-rate of the new game, or is there not a way of making the games synchronize with the capacity of the machine on which it is being played?

Um.. They do.

The store page lists the minimum requirements for the game. I don't know precisely what graphics card you have, but I've found an Optiplex 790 brochure that lists the following graphics options. This is well below the minimum graphics card requirement, so I'm not surprised you are having a bad time.

In the case of Dishonored, it uses Unreal Engine 3, thus you having a great time with it.

Dishonored 2 however, uses the Void Engine, a custom engine developed inhouse, based on a heavily modified version of the id Tech 5 Engine. When the game launched it was plagued with performance issues (it might still be?), so even high-end PCs of that time had trouble with it.
In your case, add the fact that your PC might not meet the minimum requirements, like Mortius1 pointed out, and you get the answer for why it runs poorly.

The OP is almost certainly using a faster GPU than what Optiplex 790's original 240W PSU could handle, as the minimum GPU for the first game is already many times faster than those two GPUs in the brochure.

From what I could find, the Dell Optiplex 790 was/is available in a wide variety of configurations.
Without knowing which one exactly the OP owns, it's mood to speculate whether it could handle "Dishonored 2" or not (and OP's complaints rather point into the direction of "no", anyway).

Glad you're interested in PC gaming my friend, but it's a hobby that requires some knowledge. Read some of the resources above and understand what a video card does and what you need to play games. Dishonored 2 can be demanding even today on a decent gaming PC.