I would like to add that the phrase "antique land" sounds unsual. It's odd to hear the adjective "antique" applied to the noun "land"; a more natural combination (for me) would be "ancient land" or "old land." Of course, Shelley's poem "Ozymandias" is dated 1818, so it's possible that the word combination would have passed as natural for his readers.
A quick review of the meanings of the adjective "antique" in the Oxford English Dictionary shows that the range of meanings discussed by pohnpei397 were all well in use by the time the poem was published. There's one possibly important meaning that this previous poster doesn't discuss, and that's the understanding of "antique" to mean "Of, belonging to, or after the manner of the ancients (of Greece and Rome)." If Ozymandias is indeed another name for Ramses II, which some anthologies claim, then the speaker in Shelley's poem may indeed come from (or, at least, have travelled in) Egypt, and the ruins of the statue, much like the ruins from ancient Greece, become a subject worthy of attention in the work of a Romantic poet.
To me, this phrase is meant to help emphasize on of the major themes of the poem -- the idea that all things of the world are fleeting and there is no point in trying to gain glory.
Much of the poem is devoted to telling us how great Ozymandias was (by talking about his statue and the inscription). But of course, all that is left of him or his kingdom now is this statue.
The phrase "antique land" reinforces this idea. To me, saying the traveler's land is antique means it is old-fashioned and no longer worth anything except as a memory. I think that the traveler is from what used to be Ozymandias's kingdom. Now it is just an "antique land" -- it used to be great but now it is relatively worthless.