[pixie]

Hydrogen cyanide adds to propanone in the presence of bases but not in the presence of acids. Why is this?

[Yggdrasil]

Think about what happens to hydrogen cyanide in the presence of acids and bases.  Will the reagent be more nucleophilic in one or the other?

[Arsenic]

base catalyzed :

H-CN  +   OH-    ---->  CN- +H20        step one
(acid)    (base)  

CN- + propanone ----> product            step two


in the first step the base should be used completely else it could deprotonate propanone and cause a condensation reaction.

acid catalyzed:

H-CN + H+ ----> no reaction     (* in absence of H2O)
propanone + H+ ----> activates propanone for nucleophilic reactions

although H+ activates propanone for nucleophilic reactions the lack of a good nucleophile in the reaction mixture eliminates any chance of this reaction ever happening.