Focal length data is not indexed by the Photo Mechanic catalog, so it will not be searchable. Take a look at the "Searchable fields" window (accessible from the 'Catalog' menu) to see what metadata you can search the catalog on.
As far as achieving what you are looking for, you can achieve this in a few steps.
First, I would go to Photo Mechanic's Preferences and go to the Contact Sheet section. From there, in the "Labels:" section, add the {lens} variable, so that the focal length will be visible as a Contact Sheet label.
Then, create a custom sort and use the {lens} variable as the sort criteria. This video shows you how to do that:
https://youtu.be/lBLAdszx0T8?t=36Next, do a catalog search that brings up every image in the catalog, you can do this by typing a period (.) in the Quick Search field, and then use the custom sort you just created to sort by the focal length. This sort may take a while depending on the number of images and the speed of the device on which they are stored.
Finally, scroll through the Contact Sheet and find the first image with a focal length that matches what you are looking for and click on it to select it, then find the last image, hold the shift key, and click on that one, and all images in between will be selected.