Step 1: To add a custom field with a list, go to Settings, General, and then Custom Field Lists
(If you want to add a custom field that is not a list, skip to Step 2)
Press Add List, and add a list with, for example, resellers, press the save button
Add values to the list (resellers in this case) by selecting the list and clicking ADD VALUE TO LIST
Step 2: Go to Custom Fields, and press ADD CUSTOM FIELD
For the reseller example:
When using a list, the "List" field will pop up where you can choose which list to use for the field. If you are not using a list as field type, the "list" field will not pop up.