At this time, Merlin is not able to accept a manually added coin to be linked up with a Exchange/Wallet that was connected automatically.
In order to fix this please follow these instructions:
Go to your Assets page
Click on the Coin
Press Edit Coin located in the My Asset box
Edit all transactions that have EXCHANGE/WALLET with the same name as a wallet or exchange that was connected automatically (for example, if you have "Metamask-Ethereum", rename it to "Metamask-Etherem M" or any other name that helps you identify it). To do this, press the ≡ button located to the right of the transaction then press Edit
Now under EXCHANGE/WALLET rename it to something else
Press the ✓ located to the right to save this change
And that is it. Make sure to repeat steps 4, 5 and 6 for any other transactions under this coin to match the newly named EXCHANGE/WALLET
Here is a video showing how to do it: