If memory serves a locally compact semigroup admits a left invariant Borel measure iff it has a minimal ideal which is a direct product of a group and a right zero semigroup. 

The measure is supported on the minimal ideal. 

<strong>Update:</strong> 

The result I was trying to recall is from
http://www.ams.org/journals/proc/1966-017-02/S0002-9939-1966-0188341-7/S0002-9939-1966-0188341-7.pdf. 

A locally finite Borel measure $\mu$ on a locally compact inverse semigroup $S$ is left invariant if $\mu(s^{-1}B)=\mu(B)$ for all Borel sets $B$. It is proved in the above paper that if $S$ is locally compact and left translations are proper (i.e. $s^{-1}K$ is compact when $K$ is compact, then $S$ has a left invariant measure iff it has a unique minimal left ideal. This is the same as saying it has a minimal ideal which is a direct product of a group and a right zero semigroup. You can take as the measure the product of Haar measure on the semigroup with your favorite locally finite Borel measure on the right zero semigroup.