Q:

Adjective used to mean "smellable"

An object that can be seen is visible.
Something that can be heard is audible.
What's a similar word to indicate that something is smellable?

A:

Both olfactory and olfactive have this meaning, but both words also have the more common meaning of emitting a smell, so they wouldn't be useful.
The best choice would be olfactible.  Wiktionary includes it, but as far as I can tell, they're the only ones.

A:

Olfactible (“having an odor; capable of being smelled”) seems relevant. 
You may find the wordnik list for odorific of interest; it includes about five dozen “smelling words”, the first few being  odoriferous, malodorous, heavenly, redolent, putrid, delectable,  stagnant, fragrant, smelly, noisome, noxious, fetid, aromatic, dank, musty, moldy, fusty.

A:

Odorous? (Odourous in the UK) (Same in the UK, see below)
Scented if the smell was added (Strawberry-scented envelopes, but not scented strawberries) or smelly if the smell is pungent or unpleasant (smelly cheese can be good, smelly socks aren't).

