After a lot of trial and error, here's what worked on my samsung galaxy note:
1) send yourself your certificates as an attachment
2) save them from your email by pressing the disk icon to the right of your attachment (you may need an SD card inserted)
3) from mail app, press menu and go to settings/security options/Email certificate and press the + button to add your certificate
4) select your certificate (you may have to enter your certificate password). Once you've added your certificate, you can choose the certificate by selecting it.
5) ensure the "Sign" box is checked and press "Done" to save your settings