'make it' means 'reach the goal you are striving for' - and Rose Garden is a use of metonymy, meaning White House/Presidency. So 'it' is his 'goal' rather than his 'ambition'
Q2: Why does the columnist say he'll be an orchid? Is orchid a hothouse flower? I think the columnist refers to Obama's inexperience. Right?
It's in the next sentence: For all his attempts to act like a sturdy American perennial
Yes, an orchid is a hothouse flower that can't survive the types of weather that other plants can. He is not a seasoned politician who has weathered many a storm in the cut and thrust of political life; and so far, his image has been cultivated to make him attractive to voters. He has yet to prove himself robust.
What does "cred" mean? I can't/didn't find it in my dictionary.
You'll hear/read it as 'This will give me street cred.' 'cred' is just slang shortening of 'credibility'.