Read the questbook, it tells you literally everything you need to be doing. If it doesn't tell you, then you're advancing ahead of the questbook, so try to complete a few quests that look like they're leading up to planting.
That said, link people here from now on, I'm getting really tired of people completely ignoring the questbook's superlative instructions (and the changelog, for that matter) and cluttering up the thread trying to figure out why they can't plant whatever.
Seriously, if you can't get nether wart to grow, that's understandable (in fact, I recall thephoenixlodge saying he'd fix that, since it requires darkness or summat). In my opinion, if you can't be bothered to read the questbook that guides you through the modpack, the actual mods, and many of the recipe changes, you don't deserve to waste other people's time so they can answer the inane questions that you come up with as a result.