Many people believed that the Constitution promoted slavery because of the Three Fifths Clause, the Fugitive Slave Law and the preservation of the slave trade. Douglass believed that the Constitution of the United States should be protected and respected but he certainly thought that slavery was an abomination and should be stopped. Here is what he has to say about the Constitution in relation to slavery:

Liberty and Slavery--opposite
   as Heaven and Hell--are both
   in the Constitution; and the oath
   to support the latter, is an oath
   to perform that which God has
   made impossible. The man that
   swears support to it vows allegiance
   to two masters--so opposite,
   that fidelity to one is, necessarily
   treachery to the other. If we
   adopt the preamble with Liberty
   and Justice, we must repudiate the
   enacting clauses, with Kidnapping
   and Slaveholding.

I am not sure why you keep saying that these questions are from Douglass's autobiography when they are actually from this speech he gave on the Fourth of July...

At any rate, Douglass says that lots of people think that they have to respect slavery because it is in the Constitution. But he says that the people who think that the Constitution condones slavery are wrong.  He says

if the Constitution were intended to be, by its framers and adopters, a slave-holding instrument, why neither slavery, slaveholding, nor slave can anywhere be found in it.

So he is saying that the country needs to get back to what the Constitution actually says.  He says people need to realize that the Constitution is not pro-slavery and that slavery should be abolished.