System level design is not supported in the latest versions of PSoC Designer 5.1 (PSoC Designer 5.1 onwards).
PSoC Designer 5.0 SP6 can be used to configure PSoC express devices since it supports system level design (which is a requirement for configuring PSoC Express Devices).
PSoC Designer 5.0 SP6 can be downloaded from our website through this link http://www.cypress.com/?rID=34517. Note that PSoC Designer 5.0 can co-exist with PSoC Designer 5.1. (It will co-exist with future releases of PSoC Designer also, because of system level design support).