Importing a V2 JSON file
These random V2 JSON files always make things complex. We apologies in advance.
You need to convert it to the standard V3 ("Keystore" / "UTC") file. The easiest way is using geth.
Once you download it, extract the zip. Also copy your V2 JSON file into the same folder.
(Really explicit instructions for PC and Mac are here: https://ethereum.stackexchange.com/questions/465/How To-import-a-plain-private-key-into-geth-or-mist)
Now you have to open a command line window and type in the following command:
geth account import nameofyourwalletfile
This will import your file into geth and convert it to the latest (V3) keystore file. Now you need to go find it in one of the following locations:
Copy that file, and select it on https://www.myetherwallet.com and you are good to go!
Moving foward, use this file. Keep a backup of it, and the old file just in case.