This has been tested successfully on Nokia N95 8GB and Nokia 6120 Classic.

1. Download required files from here
2. Copy “installserver.exe” to Phone Memory (C:\Data\)
3. Install “HelloCarbide.sisx”
4. install “x-plore”
5. Open X-plore (leave x-plore running, press menu to go to main menu)
6. Open “HelloCarbide”
7. Go to Options >rent a car bulgaria Menu1 and click Yes
8. swith to x-plore (press and hold menu key and choose x-plore)
9. Go to Menu > Tools > Configuration and tick “Show system files/folders”
10. Locate “installserver.exe” from Phone Memory (C:\Data\) and click Menu > Edit > Move and go to C:\sys\bin folder and click ok to move the file.
11. Restart the phone

ScreenShot
Install Unsigned Applications on Symbian Mobile

Note: Always make a backup of your phone before you do this.