First you need to install obfsproxy
package. Then configure torrc
file like @user263485 said:
UseBridges 1
# This works for Ubuntu Linux, adjust the path according to your platform
ClientTransportPlugin obfs3, scramblesuit exec /usr/bin/obfsproxy managed
Bridge obfs3 <ip:port> <key>
To obtain obfs3 bridges visit BridgeDB (step 2) and instead of clicking the inviting just give me bridges button scroll down and specifically ask for obfs3 type bridges. Restart tor and it should work, it just did for me.
EDIT:
Note that obfsproxy support also scamblesuit bridges, bridge syntax is the same
Bridge scramblesuit <ip:port> <key>
{{{ PLEASE DELETE THIS LINE }}}