Thank you, Daniel. Unfortunately, the result is the same.
I found a hint here: Getting Started Managing Oracle Fusion Middleware - 11g Release 1 (11.1.1) which states that "Many components, such as Oracle SOA Suite, Oracle Platform Security Services (OPSS), Oracle Fusion Middleware Audit Framework, and MDS, and services such as SSL and logging, provide custom WLST commands. To use those custom commands, you must invoke the WLST script from the appropriate Oracle home. Do not use the WLST script in the WebLogic Server home."
It looks as if I need to install WebCenter Portal (or at least MDS) components to the domain first, which - I have to admit - seems logical.
You will not find them in Help.
firs you need to connect to the server then try to execute this command.
Run WLST from <WCCUI_MW_HOME>/oracle_common/common/bin/wlst.sh
Please enter your username :weblogic
Please enter your password :your_weblogic_password
Please enter your server URL [t3://localhost:7001] :t3://localhost:7002
registerMetadataDBRepository("WCCUIMDSREPO", "Oracle", "localhost", "1521", "localhost.localdomain", "DEV_MDS", "your_MDS_schema_password", "WCCADF_server1")
Here is the normal output
wls:/WebUI_domain/serverConfig> registerMetadataDBRepository("WCCUIMDSREPO", "Oracle", "localhost", "1521", "localhost.localdomain", "DEV_MDS", "welcome", "WCCADF_server1")
Location changed to domainRuntime tree. This is a read-only tree with DomainMBean as the root.
For more help, use help(domainRuntime)
Executing operation: registerMetadataDBRepository.
Metadata DB repository "mds-WCCUIMDSREPO" was registered successfully.
To conclude this thread, as I assumed it was not until I installed WebCenter Portal components to the domain that a command became available (and since then they can also be found in help, Kal). In fact, it was not until I fully deployed WebCenter Portal applications to the domain (see Configuring Oracle WebCenter Portal - 11g Release 1 (188.8.131.52.0)) that the command started to work - before that if failed with an error 'cannot find MDS Repository MBean'.
And yes, Daniel, the command is called without the attribute names.