[>BKA< T Wrecks]

A) You put the func_ladder at the position where you want players to be able to climb up or down. This can be at any wall; the func_ladder will be totally invisible in game.
If you have made a visible ladder (i.e. a block that looks like a ladder), it's of course logical to place the func_ladder exactly in front of the block, so that there is no space between the block and the func_ladder.

B) I don't have Hammer installed right now... but if I remember correctly, you can select the desired block (its outlines should turn red) and right-click. Then there should be a menu, and in this menu there is an option "tie to entity". Once you have selected this, you will see a window where you can select which type of entity you want and what properties you want the entity to have.

Once again: If you want a ladder that looks like a ladder and acts like a ladder, you need two blocks: One that provides the look of the ladder and another (invisible) one that provides the function. Both should fit together like the two halves of a sandwich. The visible ladder directly at the wall, and the invisible func_ladder directly at the ladder, from bottom to top.