Understanding inputs and outputs on a multiplexer chip
The 74xx139 is a decoder or demultiplexer, not a multiplexer.

The part contains two, 2->4 line decoders.  

For each decoder, the A and B inputs select which of the four outputs will be Low when the Enable/Gate input is low.  When the Gate input is High, no outputs will be Low.

The circles on the inputs or outputs of a logic IC indicate that that signal is "Active Low" - a logic Low indicates that the signal is Active.