Import GPG key:

apt-key adv --keyserver keyserver.ubuntu.com --recv 7F0CEB10

Add into /etc/apt/sources.list

deb http://downloads-distro.mongodb.org/repo/debian-sysvinit dist 10gen

Then run

aptitude update

Then

aptitude install mongodb-10gen

Output

...
Adding system user `mongodb' (UID 105) ...
Adding new user `mongodb' (UID 105) with group `nogroup' ...
Not creating home directory `/home/mongodb'.
Adding group `mongodb' (GID 107) ...
Done.
Adding user `mongodb' to group `mongodb' ...
Adding user mongodb to group mongodb
Done.
Starting database: mongodb.

MongoDB server is configured in /etc/mongodb.conf.

By default, the database is stored in /var/lib/mongodb.

Further information

See primary documentation on MongoDB.org for installation on Unix systems.