Cannot load lwpostgis.sql

I was a PostgreSQL and Postgis user on windows, but now I have changed to Linux (Ubuntu), and I have installed PostgreSQL 8.1.4. I installed it manually, and then I installed GEOS and Postgis without problems. Finally, when I started pgAdmin and tried to load lwpostgis.sql to load GEOS functions, i got this error:

NOTICE:  type "histogram2d" is not yet defined
DETAIL:  Creating a shell type definition.
ERROR:  could not access file "$libdir/liblwgeom.so.1.1": No such file or directory found

Thanks