Chocobo Questions..

[Merryl]

Its under companions... And you'll need ghysal greens to summon your chocobo.
I believe the quest for it is in the north shroud at the stables. Ghysal greens are found at one location in each city but I usually stock up when I'm in bent branch. The mender merchant has them.

[Cons]

Its under companions... And you'll need geyshal greens to summon your chocobo.

This ^

Merchants sell greens

[Niwashi]

Its under companions... And you'll need ghysal greens to summon your chocobo.

There's two different versions of summoning a chocobo, and since the OP mentioned both chocobo quests, it's a little unclear which he's talking about, but since he included "let me mount" and "before I am able to ride", I think he's referring to summoning it as a mount rather than as a battle companion. The ghysal greens are only needed for the later.

Mount
The ability to ride a chocobo (and, although the in-game help doesn't mention it, to ride any other mounts you may have) is unlocked by the level 20 quest "My Little Chocobo". This is the one that has you collect 2,000 Grand Company seals and trade them in for a Chocobo Issuance. You give the issuance to the local Chocobokeep, he presents you with a chocobo and asks you to name it, and then you talk to him again to pick up your reward - a Chocobo Whistle. The Chocobo Whistle is a single-use item that disappears from your inventory when you first use it, but doing so places "Company Chocobo" as an available action under the Mounts section of the Actions and Traits window. From there you can add that action to a hotbar and use it to summon or dismiss your chocobo mount (provided you're in an area that allows mounts).

Battle Companion
The ability to have your chocobo fight alongside you is unlocked by the level 30 quest "My Feisty Little Chocobo" (located at Camp Tranquil in the South Shroud). It's actually unlocked during the quest rather than when you complete it (since the quest itself has you call your chocobo and fight with him). To summon your chocobo as a battle companion requires ghysal greens. You'll get a first bunch of greens during the quest, and then more can be bought in any of the three main cities.

It's also while you have him out as a battle companion that your chocobo can level up and gain skill points that you can spend to have your chocobo learn additional skills. This configuration is set up in the Companion window.

[HEC]

summon or dismiss your chocobo mount (provided you're in an area that allows mounts)

This is common problem - people complete the first quest and wonder why they can't summon it (not allowed in cities) ...