Click "Load Data" on the left and navigate to the folder that contains your Timeline data.
For Google Takeout data:
The folder structure should be "Takeout\Location History (Timeline)\Semantic Location History".
Navigate into the "Semantic Location History" folder, and then click "Select Folder" on the dialog.
(Do not navigate into one of the yearly subfolders.)
For On-Device exported data:
Simply choose the folder that contains your exported Timeline.json file.
(On non-English systems, the filename may be different -- rename the file to Timeline.json)
If you have a previously saved "TimelinePlaceCache.json" file, be sure it's in the same folder. This can speed up loading and reduce API calls.
After the data has loaded, use the date picker to choose a date or date range that contains timeline data.