A lock appears after download.
How do I seed ??

The lock indicates a private tracker, if you're downloading or have already completed the download then you are automatically seeding unless you remove the file from your list or no one needs pieces of the torrent from you, actually some private trackers still give you seeding credit even if no one is getting pieces from you.
Oh and you cannot use DHT on those, it will be greyed out.
Also keep in mind that many private trackers don't have Tixati in their approved white-list and may ban your user name or even your IP, I simply don't use those.

"If you see a Transfer-lock-icon.png lock icon next to the name, it indicates a 'private' torrent. This means DHT and PEX will not be used for this transfer. This is meant to control access to the swarm via the tracker.

If you see a Newicon.png'created' icon next to the name, it indicates that this transfer was created from local files."

http://support.tixati.com/Transfer_Properties_-_Details


The lock was added when the torrent was made and it prevents open sharing. So...

Break the lock. If I understand the process, I think the steps would be something like this:

Go to the target folder and delete any unnecessary files such as "Downloaded from.txt"
Make sure to deselect  [ ] Set Private Flag
Create a new torrent from the cleaned folder.
Add open trackers.
Upload your new torrent to those trackers.
The torrent should be discoverable via PEX and DHT now too.