Possible USE flags for NetworkManager are:

    * avahi - Enables IPv4LL support (automatic network configuration).
    * bluetooth - Enables support for Bluetooth.
    * connection-sharing - Enables support for internet connection sharing with net-dns/dnsmasq and net-firewall/iptables.
    * dhclient - Use dhclient from net-misc/dhcp for DHCP.
    * dhcpcd - Use net-misc/dhcpcd for DHCP.
    * doc - Install documentation.
    * gnutls - Enables support for TLS and SSL using net-libs/gnutls.
    * nss - Enables support for Network Security Service.
    * ppp - Enable support for mobile broadband and PPPoE connections using net-dialup/ppp.
    * resolvconf - Use net-dns/openresolv for managing DNS information. 

Set the USE flags you need and emerge networkmanager:

$ emerge -av networkmanager

After the installation you can start networkmanager with:

$ /etc/init.d/NetworkManager start

To make sure that it is started every time you boot your system add it to your default runlevel:

$ rc-update add NetworkManager default

You will also need to block OpenRC from attempting to automatically initialize your network interfaces. This can be done by modifying /etc/rc.conf:
Code: /etc/rc.conf

# You can also remove dependencies.
# This is mainly used for saying which servies do NOT provide net.
rc_net_tap0_provide="!net"