Use wildcard '*' to add more than one file at a time. For example, if you have file1, file2, etc. all in the same folder or directory; first, locate one of the files with the [Browse...] button. Then alter the file path in the box above to include '*' character (eg. file*). Wildcard '?' to blanket only one character works too.
Note: In Windows you can select multiple files and "drag and drop" them onto HistCite icon or shortcut. The program would start and load those files.