I am not sure I understand.
You say that you have created a memory channel and put markers into it. Now markers are single time points, they do not have a sampling frequency. But of course they do have a basic time resolution, which is set in the data file of which the marker memory channel is a part. So if that is the situation you need to look at how the file was created.
The other possibility is that you are using the term marker to refer to a bit of waveform and your marker channel holds a waveform, which does have a sampling frequency. This sampling frequency is set when the memory channel is created, either by copying that of another channel or by you specifying the sample interval.
I hope that helps, but do feel free to get back to me if you need more information
Tim Bergel Cambridge Electronic Design