How to add a new picklist value and make sure it's available in your page layouts
There are two steps to adding a new value to a picklist (a drop-down menu with pre-selected options). The first step is to create the picklist value and the second, if necessary, is to add the value to a record type so it's available in page layouts for that record type.
In this example, we're going to add the picklist value to a custom Grant Reporting Frequency field on Opportunity, then we'll make sure that option is available for the Grant record type on opportunity. If you only add the new value, you'll find it won't be an option on your layouts unless you also select it for the record type.
Create the new picklist value
Navigate to the field you want to edit in Setup. If it's a standard object, you'll find it under Customize > (object name) > Fields. If it's a custom object, you'll find it under Create > Objects > (object name) and the custom fields are at the top.
Click on the name of the field you want to edit. If it's a picklist field, you'll see Replace to the left of the name. If you click Replace, you'll be able to change existing records to a new value. For now, we are adding a new value so be sure to click on the name and not Edit.
Add the value(s) you want. You can add multiple new values, using a return between each value.
You can add your value to the record type here by checking the appropriate box below. This will save you from having to do the next step. For this example, we'll leave the boxes unchecked but you can save time if you check the box(es) now.
Change the order of displayed values
You can change the order the values display in your picklists. Read this article for more information.