Update 31/01/2008
You can now request the certificate yourself, straight from your phone! No need for a PC AT ALL!


Download DevReq by OPDA here
NOTE: Currently untested by myself at time of writing.
If you're on a Mac, getting your Symbian applications signed can be a bit of a nightmare. However, as long as you can get somebody kind enough to create you a certificate (.cer) file then there is still hope!
You can actually perform the signing process on your
N95 with a little application called MobileSigner by Leftup.
Here's a small tutorial on how to do it. Note: This tutorial will assume you have managed to get a certificate file.
1) Download
Mobile Signer by Leftup
2) Extract contents. You should have the .SIS file (application) and a .KEY file (generic key file).
3) Install the application to your phone.
4) With your phone plugged in in Mass Storage (or Data Transfer) mode, access your memory card and create a directory in the root, call it 'Cert' if you like.
5) Place your .CER file, and the inculded .KEY file into the 'Cert' directory on your memory card.
6) Now, you can either create another directory in the root, for example 'Install' and place your UNSIGNED .SIS files in here, or just put the UNSIGNED .SIS files in the same 'Cert' directory, its really up to you.
7) Now disconnect your phone and launch the MobileSigner application on your phone. You should have a screen like this:
8) Click on 'Sis File:' and browse to the 'Install' directory on your memory card that you created in step 6 (or the location where you put the .SIS file). Locate the .SIS file you want to sign.
9) Now click on 'Key File' and browse to the 'Cert' directory on your memory card that you created in step 4 (the directory with your .CER and .KEY files). Locate the .KEY file.
10) Now click on 'Cert File' and repeat the same process as in step 9 (but this time you want to select the .CER file.
11) Finally, click on 'Password' and enter 0123456 (only IF you're using the included .KEY file downloaded from here, otherwise enter your own password which would have been used to create the .KEY file. If you didn't create it yourself, ask the person who did for the password or just use the included).
12) Now click 'Options' and select 'Save Config'.
13) Lastly, click 'Options' and select 'Sign'. Now use either the built in File Manager application that comes standard on the
N95 or another third party one of your choice, browse to the 'Install' directory and you'll notice two files. One will end .SIS the other will end .SISX. You can easily find out by checking the file's properties, which is which, as the standard File Manager truncates the file names. Just click options > view details. The one ending .SISX is the signed one. Open it and enjoy installing your signed application.
Note: This was written by a PC user, with no Mac experience whatsoever. It
should work just fine. Should anything need altering in this How To, and you're a Mac user, please let me know and I'll alter it ;)
Additional, if you would like a .CER file, I'll be happy to help. Please PM with your IMEI number (type *#06# if you don't know by now) and I'll endeavour to get back to you with a certificate within 24 hours. All I ask is you drop back here after and hit the 'Thanks' button :D
Hope this helps.