@MikeyH Hi Mike,
Sorry, I forgot to mention one important thing: In your current REXYGEN installation, open the file C:\Program Files\REX Controls\REXYGEN 3.0.X.YYYYY\HmiDesigner\version.req and change its contents to 3.4.
The new HMI Designer requires REXYGEN v3 (I recommend using the latest available 3.0.5)
Then the Studio should automatically use the new HMI Designer. You can check it in the compile log after you compile any project with HMI.
Let me know if it helped.
Tomas