For the time being, replacements are granted automatically to units that need them. As for the special units (Black Watch, RM and so on), they need special replacement that are rarer...there are not as many scots willing to get killed in the Americas as englanders right now
We have started working on a feature for allocating replacements with a bit more of player's interference, but it will come after more crucial changes have been effected.
As for the ambush, the icon will appear normally on top of the army if you succeed in setting one up. Of course, this would be useless if nobody comes in the trap
In that field again, we are working on some features that will increase ambush likeliness...