access method "gin" does not exist

I'm trying to test the 8.1 backport of the 8.2 GIN index and tsearch2
functionality.

The patch is applied successfully (to 8.1.4, on OS X 10.4.7 w/ xcode
2.3), the build and install goes well, stop & start of postmaster is
done, but initializing tsearch2 fails:

psql -U postgres minitest <
/usr/local/src/postgresql-8.1.4/contrib/tsearch2/tsearch2.sql

...
ERROR: access method "gin" does not exist

This error occurs at the following statement:

CREATE OPERATOR CLASS gin_tsvector_ops
DEFAULT FOR TYPE tsvector USING gin
AS
OPERATOR 1 @@ (tsvector, tsquery),
OPERATOR 2 @@@ (tsvector, tsquery) RECHECK,
FUNCTION 1 bttextcmp(text, text),
FUNCTION 2 gin_extract_tsvector(tsvector,internal),
FUNCTION 3
gin_extract_tsquery(tsquery,internal,internal),
FUNCTION 4
gin_ts_consistent(internal,internal,tsquery),
STORAGE text;

Sanity checks:

1) In PSQL:
select amname from pg_am where amname like '%gin%';
amname
--------
(0 rows)

Hmm. Should I expect this query to return something?

show gin_fuzzy_search_limit;
gin_fuzzy_search_limit
------------------------
0
(1 row)

Thanks for any tips,
Kevin