To make full outer joins in DSS, you need to work with datasets that are stored on a SQL database that supports them. For example, MySQL does not support them, PostgreSQL does.
If you work with file-system datasets (or managed by DSS), it uses the H2 database for the visual join recipe. H2 does not support full outer joins.
You have workarounds for databases that does not support full outer joins: see FULL OUTER join not available with MySQL datasets