There are two ways:
1. You can c*r*a*c*k your phone's operating system. Then you can install any application.
Here you can learn more
2. You have to sign (all unsigned) applications with certificate, made for your IMEI
About the 2nd way...
You can get certificate for Your IMEI
here
Just enter your IMEI (left window) and security code (right window)
Repeat this procedure in (about) 24-36 hours and you'll get the link for certificate downloading
Then just ask uncle google how to sign symbian application with your fresh certificate
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