There doesnt seem to be a "sandbox mode" per say, but it would appear you can get/make a sandbox map that meets your criteria quite easily.
I'd look around http://arma2.filefront.com/ for what you are after.
Spawning Location
When you first spawn you'll notice some text show up on the right side of the screen. This will contain your general location, so make sure you remember this. Unless you spawn near a landmark, it's the only way you'll have any idea where you spawned.
Map & Compass
There is an in-game map, but it's an item like any other. Until you find one and store it in your inventory it won't be displayed when you press M. However, there is an online version of the map, which also contains the English names of locations. Towns will have a sign containing their name somewhere near their borders, but it will be written in Cyrillic.
There is also an in-game compass, usable with K, but like the map is only usable if you have a compass in your inventory.
The map and the compass are the most important tools at your disposable for figuring out where you are, and what direction you need to walk in order to get where you want to go.
Night time
I haven't tried this method myself, but apparently there is a way to locate the North star at night. If you find yourself wandering around alone at night this should prove useful in helping you get around.
Finding Action
Towns are the best place to find zombies and players. Zombies have a much higher spawn rate in towns (in fact they currently don't spawn in the woods at all, only near buildings). Towns are also a required stop for most players since contain the most loot. Your best bet for finding a town after you spawn is to just follow the coast in either direction. It may take some time, but you'll eventually find a town.
Something else to pay attention to is the in-game chat (by default you talk with "/", and change channels with "."). Watching this is one of the best ways to find out where players and zombies are. Players will often send messages looking for people to meet-up with, or give out warnings if their are a lot of bandits or Zombies in an area.
Best Answer
According to this forum post, the orange marks on your map are "uncomfirmed" zombies or players.
Confirming the zombie/player will turn the icon blue and give it the appropriate label (Civilian for Zombies, and Survivor/Sniper/other variation dependent on player skin choices for humans). In order to confirm the blip on your map you must be able to actually see what it is the map is pinging.
This setting is generally frowned upon, as it often shows vehicles and camps as well (making remaining hidden very difficult). It is apparently an option the server administrator can enable or disable.