I don't think it can be done. My D520 has no option to set a shutter speed. The best that we can do is try to trick the camera and hope it works out. I assume you're hoping to set a long shutter, say, for a blurred water shot?
No Olympus D-Series cameras have shutter priority. You will need to place a neutral density filter in front of your lens to keep your shutter open longer:-)
Well, the only way I've found is to trick it, as I said above. Focus-lock (half-press shutter button) on something really dark for (hopefully) a slower shutter, something bright for faster. It's not guaranteed, the camera is full-auto, so it could gladly change the apeture or ISO as well.
If it's dark, turing off the flash sometimes lengthens exposure, as well as does using the camera's 'night mode'. I like to think I've gotten fairly good at tricking mine, but that isn't keeping me from eyeing a new Canon S1 IS... =D
I also have a D-550 and I know what you talking about. Sometimes when I half press the shutter it says slow shutter but does not take a picture when I press it fully. Though I do not know why