The certificate manager application installed on the N900 currently only supports viewing of installed certificates, but doesn't allow you to add or remove any certificates.

It took me a while to figure out how to import an X.509/SSL client certificate which is actually quite easy (I didn't find anything about this in the user guide). So I'm writing this in case other people have the same issue.

• If the client certificate/key is already installed in your PC's browser, export it into a PKCS#12 file (.p12 file extension). Pick an appropriate password when exporting as this will be the one used on your N900 as well.
• Copy the file to your N900, e.g., store in the Documents folder.
• Open the File Manager and click on PKCS#12 file. This should start the import dialog: enter your password, select if you are importing the certificate for Web sites, email, or WLAN, and confirm the installation.

The client certificate and key should then be available (the same works for server certificates as well). You may need to restart an active browser or email application for the new certificate/key to be available.

The Certificate Manager application should now display your imported certificates.

Vote for the following bug to get the missing features added to the Certificate Manager:
https://bugs.maemo.org/show_bug.cgi?id=6738


cheers...
Lars