The left side shows directories, the right side shows datasets.
Click a directory name to display the datasets available in that directory. Double-click to change to that directory. The "." directory is the current directory. The ".." is the parent. All others are subdirectories.
Click on a dataset to only work with that dataset, or click nothing to work with all datasets simultaneously. Click on a column header to sort by that column (dataset name, instrument, number of runs in that dataset, earliest run, latest run, comment).
Click "Apply" or "Ok" to categorize all files in the dataset/directory selected. "Apply" leaves the choose window open, "Ok" closes it. Click "Cancel" to close the choose window.