The command is:
GetSizes NODESELECT MD5
(The NODESELECT part is optional but probably desired.)
It will add an MD5 column on to the lister where you can read the data. If you run the command with a file selected (or from a file's context menu) the only that file (or files) will have its MD5 calculated. If you run the command when nothing is selected then everything below a certain size will get calculated. The size can be increased via Preferences - Misc - Advanced.
If you instead want the checksum as a text string in the clipboard then see this FAQ, the last two examples in particular.