Nefarox, Overlord of Grixis
Nefarox, Overlord of Grixis is an easy one to locate. It's right in Nefarox's name: Grixis! So Nefarox comes from the plane visited in Shards of Alara block, presumably before Alara Reborn, where the five shard-planes combined to reform Alara.
Krenko, Mob Boss
Krenko, Mob Boss is another easy one. In the flavor text, Krenko is described by Agmand Sarv, Azorius hussar:
"He displays a perverse charisma fueled by avarice. Highly dangerous. Recommend civil sanctions."
—Agmand Sarv, Azorius hussar
Azorius is one of the guilds of Ravnca, so that's where Krenko's from. The plane was visited in the original Ravnica block, and we're going back there this fall with Return to Ravnica.
You can learn more about Krenko in this week's Uncharted Realms.
Talrand, Sky Summoner
After Talrand conquered the depths of Shandalar, his ambitions drove him skyward, joined by servants whose drive and curiosity equaled his own.
Talrand, then, is from Shandalar, the home of the five rings of Magic 2013.
Yeva, Nature's Herald
"Nature can't be stopped. It rips and tears at Ravnica's tallest buildings to claim its place in the sun."
Odric, Master Tactician
Luckily, Odric, Master Tactician has a visual cue. See the hilt of the sword Odric is holding? That's the symbol of Avacyn! Arcana has covered this symbol in detail (twice, in fact), so that definitely means that Odric is from the plane of Innistrad, as seen in Innistrad!
You can learn more about Odric in the Uncharted Realms from June 27, 2012.