First try - from wiki
~ $ root

BusyBox v1.10.2 (Debian 3:1.10.2.legal-losso3-0m5) build-in shell (ash)
Enter 'help' for a list of built-in commands

Nokia-N900:~# apt-get install preenv
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package preenv
Nokia-N900:~#

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2nd try

~ $ sudo gainroot
Root shell enabled


BusyBox v1.10.2 (Debian 3:1.10.2.legal-losso3-0m5) build-in shell (ash)
Enter 'help' for a list of built-in commands

/home/user # apt-get install preenv

Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package preenv
/home/user #

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so as you can see... nothing works ..

I already did a apt-get update