Donalbain is not with Malcolm’s army.
Donalbain is somewhat of a mystery. He is not chosen as Duncan’s successor, since he is younger. He flees to Ireland when his brother goes to England to gather an army, and he doesn’t come back to Scotland to fight Macbeth. He is either a coward or incredibly self-centered.
When their father is killed, Malcolm and Donalbain flee. For this reason, they are suspected of complicity or of being the ones who killed their father. This was all part of Macbeth and Lady Macbeth’s plan, of course. Donalbain seems to think that they will be better off separating.
To Ireland, I; our separated fortune
Shall keep us both the safer: where we are,
There's daggers in men's smiles: the near in blood,
The nearer bloody. (Act 2, Scene 3)
It actually makes sense for Malcolm and Donalbain to flee, even though they weren’t involved, because the person who killed their father clearly did not intend for either of them to be king. Whoever it was might come after them next. Macbeth makes no effort to kill them, but the plan works well because they flee after their father is killed.
We do not really know what is going on inside Donalbain’s head, but we do know for sure that he is not with Malcolm’s army, because Lennox tells us this.
Who knows if Donalbain be with his brother?
For certain, sir, he is not: I have a file
Of all the gentry: there is Siward's son,
And many unrough youths that even now
Protest their first of manhood. (Act 5, Scene 2)
Malcolm, on the other hand, was very patriotic and kingly. He went to England, raised an army, recruited and tested Macduff, and came at Macbeth with everything he had. He was clever enough to hide the numbers of his army with tree branches. His army was liberating Scotland, and Macbeth’s was harboring a madman. Soon enough, Malcolm was victorious and took his rightful place as king of Scotland.