If you previously installed, ran, and created a new account using the geth client and then installed Mist / Ethereum Wallet, your accounts from geth typically show up in Mist / Ethereum Wallet. This is because the keystore files (your encrypted private keys, in JSON format) for accounts created via geth command line and Mist's GUI are stored in the same place.
However, on Linux, the folder may have changed in the past few months. While researching this answer, I found some conflicting reports of where accounts' keystore files were saved on Linux.
In addition, if you were using the eth client (C++, cpp-ethereum) instead of geth, that may also result in accounts being in different places.
Hypothetically, all keystore files are saved at ~/.ethereum/keystore on Linux, but it seems like occasionally this isn't the case.
So, in order to get the account you previously created via geth into your Mist, first locate where your Mist is storing the keystore files. Go to the menu bar and select "Accounts" -> "Backup" -> "Accounts". This will open a folder.
If you already know where your geth keystore files are, or you have a backup, simply copy that keystore file into the folder you just opened via Mist and Mist will read it.
If you have no idea where the keystore files are, you can try doing a search on your computer for either utc--2016 or utc--2015. The full filename for the keystore files looks something like: UTC--2016-04-18T03-24-17.849Z--8688b2e109b93a77578eea43b3ebf6bc4be996f5.
I can confirm on my Mac that searching for utc--2016 displays all my keystore files that were created in 2016 that are not in the (hidden) default location for the keystore files. Explicitly searching the hidden (on Mac it's referred to as "system files") returns search results for the keystore files Mist is reading.
The JSON is called an ABI.
You do need the source code, as you have, and one way to get the ABI is to paste it in Remix IDE, compile it, and then click the 'ABI' button to copy the ABI into your clipboard.
Best Answer
Summary
Fill in the JSON Interface field in Mist (Ethereum Wallet) with the contract ABI (Application Binary Interface) that is listed in Ethereum Alarm Clock 1.0.0 documentation - Scheduler ABI and the contract will be loaded.
This works with versions 0.5.1 and 0.6.2, but NOT with 0.7.2.
A bug report has been lodged on github.com - Cannot load Ethereum Alarm Clock Scheduler Contract in 0.7.2 . Working in 0.6.2 . #515 . This bug has been fixed in 0.7.6 (but the Mist freezing issue when The DAO contract is in the contracts watchlist is still a problem).
Further Details
Response to additional question about checksum
The security features refers to the checksum on the address. Before the checksum feature was introduced, addresses like
0x6c8f2a135f6ed072de4503bd7c4999a1a17f824bwere all lowercase. I entered this address in etherchain.org and copied the equivalent checksummed address which is0x6C8f2A135f6ed072DE4503Bd7C4999a1a17F824B. Both versions should work in the Ethereum Wallet.Here is the blank screen in Ethereum Wallet 0.7.2
A bug report has been lodged.
Here is the working screen in Ethereum Wallet 0.6.2
A similar screen is also displayed in 0.5.1
You may want to try 0.6.2, and check the network
You might want to try downloading the old 0.6.2 version of Ethereum Wallet and see if you can load the contract in your screen. The issue you are encountering may be a bug in the new version.
And check that you are using the Main network (menu Develop -> Network).
While submitting the bug report, I when through the recommended steps to check. One of these steps was to load the console (menu Develop -> Toggle Developer Tools -> Wallet UI). The 0.7.2 console showed the message
Error: CONNECTION ERROR: Couldn't connect to node on IPC. (...).