The tub has to be level to work correctly. You can mix some mortar shovel some underneath the tub and then set it in place and mush it down so that it is level. Make sure you get it level right to left and side-to-side. That won't help the uneven floor, though. If you used a drop-in tub (i.e., one without an apron - it still sits on the floor but has no finished side wall), the wall you put up could be made to fill in without problems.
If you want to tile the floor, it needs to be flat. It doesn't need to be level (but it is nice if it is!). How is the rest of the floor? Depending on the floor, one way to make the floor flat is to use something like self-leveling concrete. This special stuff is mixed up so that it is like cream, you pour it out on the prepared floor, it seeks its own level, and voila, you have a flat floor! You might have to push it around into the corners, but other than that, that's it. Kind of expensive, but it works. I just got done doing that with mine, actually fairly easy, except that it is heavy. You can walk on it in 4-hours, tile it the next day. Hope this helps.
Now, you really should figure out if there is a structural problem that caused the floor to be uneven first. Patching it without figuring that out and fixing it is opening you up for failure. They could have cut one too many joist openings for the plumbing or heat ducts, and the floor is unstable, and that is why it is uneven.
You can get some great info here on plumbing. www.johnbridge.com will give you some great info on tiling, if you decide to go that route.