In Henry Longfellow's The Slave's Dream, what is the main theme and what's the blood red flag? What makes the slave smile in his dream, what's...

The main theme in Longfellow's The Slave's Dream is the nature of freedom.

This is evident in the way the slave is free whilst dreaming but not when he's actually awake.

To keep things organized, I'll address your other questions one at a time: 
 
The blood-red flag mentioned in the poem is not actually a physical flag at all. Instead, it's a simile used to describe what the flock of flamingos before him looks like. The flamingos also serve as guides for the slave while he sleeps. 

The flamingos are brought up when Longfellow says, "Before him, like a blood-red flag, / the bright flamingoes flew; / From morn till night he followed their flight, / O'er plains where the tamarind grew" (Longfellow, 25-28). 
 
Towards the end of the poem, the environment comes alive. Both the forest and the desert are personified and each, respectively, is shouting and crying (triumphantly) about the freedom that exists there. The slave then smiles knowing that, within his dreams, he's free. 
 
The poem takes a bit of a turn when Longfellow then says, "He did not feel the driver's whip, / Nor the burning heat of day; / For Death had illumined the Land of Sleep, / And his lifeless body lay / A worn-out fetter, that the soul / Had broken and thrown away!" (Longfellow, 43-48). 
 
Basically, when the slave has this wonderful dream about freedom and being back home with his loved ones, he simply doesn't wake up again. That said, he's not saddened that his life has come to an end. 
 
Because he's dead, he no longer feels the pain of being whipped and doesn't suffer from the sweltering heat either. In fact, death has permanently freed him because, as it is implied by the speaker, freedom transcends the physical realm.