If you want to perform a task on a z/OS system, for example updating a system configuration or downloading a product set description, you must be logged on to this system.
If you are using the same ID and password for logging on to the msys for Setup workplace that you also use on all z/OS systems that you need to log on to, then you can instruct msys for Setup to do these logons for you whenever required (see the General panel of the user property sheet).
In all other cases you are prompted to log on to the z/OS systems explicitly as required.
To explicitly log on to a system, select that system in the resource tree and then click File --> Log on to System....