Draw out the alternating charges of the various resonance states with and without a proton attached. You will find that the oxygens carry a partial negative, the carbonyl carbon are partial positive, and the alpha carbons are all partial negatives.
When you deprotonate at the central carbon, you get three major resonance states available. One with the negative charge on the central carbon and two with the negative charge on an oxygen. If you try that with the external carbon deprotonated, you can only get two major resonance states. That means, due to resonance, the central carbon is most acidic because the conjugate base of that acid is most stabilized.
A nucleophile is a compound that is 'attracted to the nucleus' of the other atoms. Meaning it is attracted to positive charges. For that to be true, it must be negatively charged. The most positively charged position on acetylacetone are the two carbonyl carbon positions. The resonance structure that creates a positive charge on the carbonyl carbons are not very stable, and therefore contribute very little to the overall positivity of the carbonyl carbon. That's not to say it won't happen, but it would be harder for a nucleophile to attach to the carbonyl carbon.
What would be easier to do, is for an electrophile to attach. Electrophile are 'attracted to electrons' and therefore, attracted to negative charges. To be attracted to a negative charge means it must be positively charged. The most likely place on acetyl acetone it would attach to would be the central alpha carbon due to the resonance states.
The compound, acetylacetone, is also symmetric. Each carbonyl carbon is identical along with each outside alpha carbon.
