I'd recommend testing your field extractions using the rex command in a search before adding them to the extractions page. Just enter your search terms, followed by | rex "your regular expression field extraction". I usually also follow it with | stats values myFieldName just to make sure I pick up only the values I wanted and don't have to adjust my regex. So for instance, if I were extracting browser from a log, I might use the following search to test my field extraction:
your search terms | rex "userAgent=(?<browser>[^(]+)" | stats values browser
I typically create my field extractions by editing the props.conf directly, but I'm a shell bigot. (8->) When ever I cannot see my fields defined in the Fields Sidebar, I realize that I have failed to create metadata for my fields. As you are using the Web UI, I am not certain what might be doing this for you. Do you have access to the file system? Check the metadata/local.meta file in your app's etc/apps directory and assert that it has permissions set for your fields.