The tiles with a 'P' inside are 'Penrose tiles'. These are tiles which have additional matching rules which permit some combinations and forbid others. Penrose tiles have (optional) markings on them which enable you to see which combinations are permitted and which are not.
To select a tile, click on either the tile or the radio button next to it.
To position a tile next to another tile, click close to but just outside of the edge which you want the new tile to join. Easy Tiler looks for the three closest points and then tries to find a suitable edge for the new tile to join. It sometimes happens, particularly when clicking inside a concave group of tiles, that Easy Tiler cannot find the right edge, in which case it will either put a tile in the wrong place or go beep. If the former, delete the last tile using the appropriate button in the corner of the screen and try again. You will usually find a place where you get what you want. In the last resort, delete a couple of tiles and put them in again in a different order.
To delete a tile which is not the last in the list, double-click inside it using the RIGHT hand mouse button.
In the case of tiles which have unequal edges or matching rules, Easy Tiler will automatically rotate the tile until it finds an appropriate edge. Where there is an alternative, you can rotate the tile manually by clicking on the 'Rotate last tile' button.
In the case of Penrose Kites and Darts, it sometimes would appear that it is possible to place a tile in two different ways. In fact, according to the rules of Penrose tiles (which are built in to Easy Tiler) there is only ever one way to place the tile against a given edge (though there is sometimes a choice as to whether you place a kite or a dart).
If you want to add a tile to a new area of the screen - so that you can compare two tilings side by side, for example - press the CTRL key down as you click on the screen.