To use this app, you have to submit a valid level.dat file of your savegame first. Either click the "Load.."-Button to open a file browser, or use an external file manager to drag and drop a level.dat file into your web browser.
Once the level.dat file is loaded, the spawn position will be shown at the top, and the spawn chunks will be highlighted on the map. Chunks marked in light blue will not receive any entity updates if there is no player nearby, while those in dark blue always do. Both types of chunks will always be loaded, however.
To scroll, use your arrow keys while your mouse cursor points at the map, or move your mouse while holding down down the left mouse button. You can use the slider below the map or your mouse wheel for zooming. The lower inputs allow you to go to a specific point (e.g., your base) of the map and set a marker there. You can always remove and add the marker by double clicking on the map. The "Save Map" button allows you to save the currently shown map as png image file.