In earlier versions of PostgreSQL (7.4), tsearch2 was a contrib module. With PostgreSQL 9.*, it was included in the standard PostgreSQL package with minor naming changes (e.g. the function "rank" became "ts_rank"). PostgreSQL 9 included a backward compatibility module named "tsearch2". Newer OpenACS installations (at least 5.9.0 or newer) do not need the compatibility package. In PostgreSQL 10 the tsearch2 compatibility package has been removed.
On new OpenACS installations for PostgreSQL, install the tsearch2-driver package via "/acs-admin/install/" and mount the search package under "/search" via "/admin/site-map" if necessary.