Here is a link to the manualslib.com website, where you can read an online copy of the Casio CTK-601/611 manual. Click on
TABLE OF CONTENTS and scroll down to
Changing MIDI Settings, and click on it. The next several pages will show you the available settings. Pages are shown in pairs in highly condensed format. At the upper right corner of each pair of pages is a plus sign in a magnifying glass icon. Click on that and it will give you an on screen magnifying glass to enlarge parts of the image. Basically you need to press the button you have been pressing, but until
GM Mode shows in the display instead of
Trans, as you have been doing. You can now scroll through the keyboard's available MIDI settings.
BUT . . . . the manual only confirms what I already knew. The CTK-601/611 does not transpose its MIDI output. Very few, if any, of those CTK models ever did. You will need to go to your receiving device or program and adjust its MIDI settings to make the transpose adjustment that you need.
https://www.manualslib.com/manual/358030/Casio-Ctk-601.html#manual
In the event that your receiving device or program does not allow transposing, here is a link to the sourceforge.net website where you can download their MS-Windows based MIDI router, which can do any number of different MIDI filtering and routing functions, including transposition. All of its effects can be applied on a channel by channel basis. It will make a handy addition to your MIDI toolkit. I have used it for several years:
http://midirouter.sourceforge.net/
Since the CKT-601/611 has only 5-pin-DIN MIDI connections, this arrangement may not be economically feasible, as it may require you to invest in at least one, and possibly two 5-pin-DIN-to-USB MIDI Adapters, depending on your set up. Reliable ones go for about $20 to $30 USD or more.