I created a field outside the portal.
In what table is that field defined? The portal's table or the layout's table? I would guess from what you have described that it is defined in the portal's table. If so, moving the field into the portal row and then selecting a value in the field should enable the value list to display values--though this is probably not the result that you had in mind.
Please describe your layout design, the relationship and your value list setup in much more detail and we'll see if we can't spot where things went wrong for you. Also tell us how you want the conditional value list to work for you.
Any field from a related table not inside a portal will only reference date in the first related record. In an unsorted, unfiltered portal, this will be the same as the record shown in the first portal row.
If you want to select a single value in that field and then have it control what values appear in the value list in every row of your portal, you should define this field to be a field from the Layout's Table Occurrence instead of the portal's. You'll also need to update your relationships and value list setup to work with that change.
You may have seen these links already, but here's my "info dump" on conditional value lists:
There's a lot of overlap between the first two links so you can read one and skim the other. The demo file gives you several examples of different conditional value lists so you can examine them to see how they are set up. If you are using FileMaker 12, you can open this demo file from your File menu to get a converted copy of the demo file that you can examine in FileMaker 12.
The last link discusses how to set up a chain of conditional value lists where the value selected in each conditional value list controls the values listed in the next value list.
Forum Tutorial: Custom Value List?
Knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list
Hierarchical Conditional Value lists: Conditional Value List Question
Feel free to post follow up questions here if those links aren't enough to get your value lists working correctly.