The short answer is that you can't save. They didn't implement a save and unless you're going to get on-board for modding this project, it won't likely have something like that. The single player in this game seems to take each map individually as a whole and does not maintain a state across maps aside from some of the scoring mechanics.
In the game docs, it clearly states that this game does not have saves and that the developers will not add them. Sorry. Here's the relevant quote, bolding is my own:
Classic SP works differently from most FPS games that employ a savegame based system. The developers believe that the problem with savegames is that they take away any tension in gameplay; since you play without fear because you can make frequent saves, and when you do have to reload, its just an annoyance (or frustration, if you forgot to save for a while because it was going so well). Because of this, savegames will not be added to the game.
Instead of savegames, Cube 2: Sauerbraten employs a novel system based on respawnpoints (not to be confused with checkpoints, which are just an annoying version of savegames). The major annoyance in other games comes from having to repeat the same thing, here, you can die, and still never have to repeat the same gameplay again, yet you still have strong motivation not to die. This brings back the tension in gameplay, without the frustration.
The way it works that when you die, the world stays AS IS. Dead monsters stay dead, and alive ones just continue at their current location. You respawn, as if it were DM, at your last respawnpoint. Respawnpoints are entities placed by the level designers in various spots throughout the level, and the game remembers the last one you touched. You can touch these more than once.
When you respawn, the evil monsters will have stolen your armour, and most of your ammo (currently they take 2/3rds, unless you have 5 or less, in which case they don't take anything). On the plus side, you will have all your health back (and you have kept any healthboost powerups!), and your starting supply of pistol ammo (see, the monsters are evil, but fair). Even though you are punished for dying, you are never stuck, since even if a group of monsters is very hard to overcome, they will be easier every time you try, since the dead ones stay dead (and the hurt ones stay hurt!). You may have to work with your pistol more, but that is part of the tradeoff.
An additional motivation to not die, is that deaths are the most important component in reaching a good score on a level...