Resolutionator only displays resolutions that the display tells the OS it can do. If you don't see the particular resolution you want to use, there are a couple things you can try.

First, open Resolutionator's preferences, and make sure both "Show non-retina resolutions" and "Show silly resolutions" (in General) are checked. If they aren't, check them and activate Resolutionator again to see if your desired size is now listed. If it is, great. If it's not, try the next step…

Open Resolutionator's preferences again, then hold down the Option key. This will reveal a "Show all resolutions" checkbox; check it, and try Resolutionator once more. If your desired resolution is listed, you're good to go. If it's not listed, there's nothing we can do to make it appear, because we're only showing you what the monitor is telling the OS it can do.

If you need more control over resolutions, you might try SwitchResX. It's costlier than Resolutionator, but offers very low-level control that may enable you to use additional resolutions.

Note that it's possible to get in a situation where your screen goes black, even after a reboot, because of the low-level of control offered by SwitchResX. You can recover from this problem, but it can be a bit tricky. This is the tradeoff for accessing the display at a lower level than we offer in Resolutionator.