Present intead of Future
"La tengo mañana." was translated into "It's tomorrow".
Why not "La tendré mañana."?
2 Answers
I don't know where you got the translation, but there must be other context involved. "La tengo mañana" would be literally "I have it tomorrow". As Mike pointed out, it is quite common in Spanish to refer to the near future using the present tense. Another common construction for this is "ir a". In your example, this would be "voy a tenerla mañana". Of couse, you can also use the future tense.
Either one would be okay (though it would be tendré, not tendrá), though a lot of times, if it is in the near future (like tomorrow), it is common to use the present tense.