How do I make my (linux) phone open *.cer files?

Using a phone with WIFI and GPRS (Motorola ZINE ZN5). It's important for security reasons to have installed appropriate certificates... The problem is I can't make my phone open the *.cer files to install the certificates. This means that I cannot use certain programmes or connections.

I'm a novice to linux so I really hope you can help me. How do I solve this problem?

//Jonas