Unsigned Thunderbird spell extension

[Giuseppe Giannuzzi]

Hi all,

I'm cannot install spell-(lang) for Thunderbird.
I downloaded .xpi file from 
http://dictionaries.mozdev.org/installation.html and
when I try yo install from file in Tools/Extensions/Install I see file 
marked as Unsigned
and text tell me that I should only install software that I trust ...
Clicking anyway on Install now, the installation looks like successfull 
but I cannot find
the extension.
Link above say:

*Mozilla Thunderbird:*
*1.* Download the dictionary you want to install (right-click, /Save 
Link to Disk/...).
*2.* In Mozilla Thunderbird go to /Tools / Extensions/ and click 
/Install/. Select the dictionary you downloaded and click /Open/.

Users having trouble installing, try the following:

      1. Copy the link from the Installation page
      2. Select the /Tools/ menu
      3. Select the /Extensions/ menu item
      4. Click /Install/
      5. Paste the link into the filename textbox
      6. Click /OK/
      7. Click /Install Now/ in the /Software Installation/ dialog after
      the 3 second countdown 

The problem is that clicking on Install I cannot paste the link anywhere!!!

Any help?
10x

Beppe