Quick version
If you bought it through Steam, check the Downloadable content (DLC) option from the in-game menu to confirm it's authorized for you (it should be). In that case just start playing and it will download while you're in-game. Note: My first character didn't get the starting bonuses at the beginning (Golden Guns and Eridian Artifact) since the DLC wasn't loaded immediately on launch, though they showed up later. My second character did get the bonuses from the start.
Details
When I first started the game this morning I checked for DLC in the in-game menu. It listed "Borderlands 2 Premiere Club" as purchased but not downloaded. Steam also showed it as purchased: from the game library right-click Borderlands 2 and choose View Downloadable Content.
After hunting around for a while in B2 and Steam looking for a way to install it I decided to just start playing. Now it shows as purchased an installed, and while playing I was notified via pop-up that I have the golden key. So apparently it installed itself while I played. I didn't need an activation key; I purchased it through Steam a couple of days before launch. Also, in Steam I now have two entries: Borderlands 2, and Borderlands 2 Premiere Club. This is the same way Steam DLC has worked for me in the past.
How to get the starting bonuses
My first character didn't get the two starting bonuses initially: a class-specific Eridian Artifact and a set of golden guns. Later in the game they showed up, though by that time the golden guns were no longer valuable (the artifact is still good). This happened because the Premiere Club DLC was still downloading at the time I started my first character.
If you want to be sure of getting the bonuses either:
create a temporary dummy character, wait for the Premiere Club DLC
to install, then create your "real" character; or
wait at the in-game menu until the DLC installs before creating your character.
Fair warning: I'm not sure option 2 works since my process was different, but it's worth a try if you don't want to create a temporary character.
I fought Pimon & Tumba both while on True Vault hunter mode. If I remember correctly Pimon is a stalker & Tumbba is a skag. They can also spawn in Normal Mode (Thanks, @SheDevil).
They appear on the field outside of preserve cells (where bloodwing was kept).
I will suggest weaken them both and take them down with a grenade or something.
Best Answer
You can get crystals from both Hyperius and Master Gee the Invincible. Both of these fights are more tactical than complicated, and can be really difficult to do without an experienced team. Both of these bosses have quests that lead to them, after you beat the main story for the DLC.
Each boss drops ~10 crystals each. This is painstakingly low, as a weapon in the Seraph Vendor will cost you 120 shard, where as the Relic (always the Item of the Day) will cost you 50 crystals. The Relic is an upgraded version of the Blood of Terramorphous, giving both health regen and increased max health. The weapons are, stat-wise, pretty useless for the price, but can be a (slight) upgrade to the weapons you may have now.
Every one of the pink "Seraph" items have a red text, including the relic and Shield. As of yet, nobody has figured out what the red effects are.
Note that, since a recent update, you'll have to defeat these bosses on True Vault Hunter mode to get crystals. They'll be easier but won't drop crystals in Normal Mode.
Voracidous the Invincible from the Sir Hammerlock's Big Game Hunt DLC and the Ancient Dragons of Destruction from the Tiny Tina's Assault on Dragon Keep DLC also have a chance of dropping crystals upon defeating them.