If you didn't use the big green "download" button at
http://tiddlywiki.com
to download TW, it may be broken. So opening a TW and use the browser
File:Save As menu doesn't work.
After editing, to save the TW, you need to use TW's internal save
mechanism (see TW right sidebar: Save Canges).
If you use FF or IE saving should just work.
If you use other browsers, you will need the TiddlySaver.jar (java
applet) described at
tiddlywiki.com. If you search for
"TiddlySaver.jar" you'll get some more links.
hope this helps
-m