Any time you select an ICD-10 / diagnosis code for a patient through Encounters -> Assessment, that code will move to your Common list which is the default tab that this screen loads with. If you cannot find a code in your common list, the full list is in the system and you must click on the "Adv. Search" tab to find it. You can search by code or by description (at least the first 3 characters). Remember these are provided by CMS, so the full description might be a little different than how you know it. You should try to use less words when using the description.

The RED records represent non-billable codes, meaning to add to an encounter, you must get more specific. 

You can click on the Note icon to see these codes in a tree view which can help see related codes to more specificity:

Tree view where you can use the Back to Search button, or double click to select a code:

If you would like to use a different description for a code, you can add  "User description" from Admin => Encounter Related Lists -> Diagnosis Codes Lists. Search for the code, the simply put your new description in the User Description cell. For example, you might know M54.4 family of codes which is Lumbago which is "Lumbago with Sciatica" as Low back pain, you can add this description as such:

This will allow you to search by your user description from Assessment screen: