(1) At the top of the program window, below the menu bar, is a toolbar that contains new, open, save, etc icons -- one of them shows the tileset list. Uncheck the "active" boxes next to unwanted tilesets, and add the era one (you need to know the filename). After you've added it, check the "main" tileset box. Note: only one 'main' tileset is used.
(2) After you've placed (left-click) the selected tile(s), click and hold down the right mouse button, only releasing once you're ready to place the tile(s). Repeat as desired.