accessing the secure element/MIFARE area, i.e.
requires the MIDlet to be trusted by the phone.
On a real phone (Nokia 6131 NFC and Nokia 6212) the only way to get your application trusted is by signing it with a valid Java code signing certificate. This can be bought for instance from Thawte.
On the emulator you can bypass this limitiation by changing the trust level in the emulator's settings. In the emulator window select the menu Tools -> Preferences... On the MIDP tab there is a setting called "Security domain". If you set this value to either "trusted" or "maximum" your application should work again.