Yes, the resistor is to prevent malfunction of the led - it must have the current limited. Without it, you have voltage sources (+5V) and voltage drops (for instance, 1.7V of a red led segment) but nothing to limit the current from the difference (3.3V).
I cannot answer the other question because not enough information has been provided. The 2-bit address signal is what drives the 74LS139, it is what also selects the digits. If you don't have something like that, you are going to need an oscillator and a counter.