Did you indicate the file's location at the "Clipboard / Save" section of Rikaichan's settings (Tools => Add-ons => Rikaichan => Settings => Clipboard / Save => Save file) ?

As a little tip, a shortcut at your quick launch bar - or wherever you deem it useful - pointing to the textfile used by Rikaichan can be pretty useful if you're going to use it frequently.

I don't know about Mac, but in Win rikaichan uses a plain text file - .txt -, so probably it's a format problem.

And yes, that feature isn't too well implemented in Rikaichan, which is a shame...

If you like it, you can have a spreadsheet software open when you use Rikaichan, use "c" to copy Rikaichan's results into the clipboard and then paste them into the spreadsheet. That way you can make a nice glossary, and each field -word, reading and meaning- will get pasted into their respective columns as if by magic.

I was just having the same trouble (Win XP and Thunderbird 17.0.4) but I seem to have got it working- go to the "clip board and save" tab under the options, then where it says filename click on the browse button and create a file from scratch.  Do not put an extension, just call it whatever. 

Then when you press the s key, with the file closed, it actually saves it. If you have the file open, it will say that it saved it, but you don't see it. Then if you close it and open it again, it is there! Weird!

Anyway don't rename it afterward or anything because it will not work, at least don't add and extension.  I used word pad to open it.

Now I just need to find a way to automate the actual learning of the new vocab list...