The Map View is where you do all your drawing.
The mouse buttons work as follows:
You can scroll around the map view using the arrow keys. If the arrow keys don't seem to work, choose the Selection Tool from the toolbar, click anywhere in the map, and then try the arrow keys again. If you hold down the Shift key when using the arrow keys on your keyboard, it will go a page at a time instead of a cell at a time.
As you move the mouse around, the editor shows the coordinates of the square the mouse is over to the right of the toolbar.
As you scroll, the View Origin changes. When you start the Map Editor or open a new map, the origin is set to (0,0). When you scroll around, the origin changes - this means that the map coordinate of the upper-left corner of the view can be something other than (0,0). If you want to "jump" to a particular origin, you can use the Origin Widgets — type in the new X,Y origin and hit Enter, and the view will jump to that location in the map.
By default, the editor shows white "grid lines" at the edges of every cell. You can turn them on or off by going to the View menu and clicking the "Show Grid Lines" checkbox. The grid lines are useful for helping you place objects, but turning them off gives you a better feel for what the map really looks like in the game.
When the Map Editor displays a bounded map, it has to show something in the areas that are "out of bounds" (i.e. outside the edges) for the map. The default for most maps is "terrain/empty", which is a solid black tile. Bounded maps are able to override the default out-of-bounds terrain in the Map Properties dialog. If you try to perform a view operation in the out-of-bounds area, it will fail and the Map Editor will beep at you.
The Wyvern engine has the ability to display "terrain borders" around terrain tiles. You can turn this feature on or off in the editor using the View->Terrain Borders checkbox. The terrain borders will still show up in the game - this feature just lets you see what the map looks like with or without borders. It can come in handy when the borders are overlapping a square and you can't tell what the terrain type for that square is.
To configure the terrain borders for the terrain in an individual cell, simply right-click on the terrain in that cell to bring up the Popup Menu, and select Edit Terrain to go to the Edit Terrain dialog box. You can turn borders on and off for that terrain, change the "priority" (the layer the terrain draws in), and more.