The N70 is an older SymbianOS 8.x device. The security application security model was overhauled in SymbianOS 9.x and requires applications to be signed (placed in trusted security domains) before being able to access certain protected device functionality/api's.
Thats why apps like FExplorer I previously mentioned and RotateMe require signing to be installed on SymbianOS 9.x devices such as the N95, E series etc.