The manuals for DB2 z/OS are located here:
IBM DB2 for z/OS - Technical Resources - United States
DB2 z/OS Commands are documented in the Command Reference manual, but DB2 z/OS and DB2 Linux, UNIX, Windows are two different products and the commands are not the same. So your assumption that there is a DB2 z/OS command for design advis is false, although there may be other IBM tools or 3rd party tools to accomplish tuning (Don't assume that all IBM tools are free and included with DB2 z/OS).
There is an option of ISPF panel for DB2 were you can enter the DB2 z/OS commands interactively. I believe it is option 6 on the DB2 panel (but not sure). You can also submit the commands via a batch job.