The A610 is supported — stable port, for firmware versions 100d, 100e and 100f. So the "no build" message from STICK almost certainly means it couldn't identify your firmware version, not that the camera isn't supported.
CHDK builds are made for exactly one firmware version, so you need to know yours first: put an empty file named ver.req in the card root, start the camera in playback mode, then hold FUNC./SET and press DISP. The version shows on screen. (Details on the wiki FAQ.) Then get the matching full package from the downloads page — install the full package the first time, the small one is only for updating an existing install.
About the missing button: on these older cameras you load CHDK through the camera's "Firmware Update..." menu item in playback mode. It doesn't actually update firmware — it just makes the camera think it is, and loads CHDK into RAM instead. Nothing is flashed and nothing is permanent. You have to repeat that at every power on; the alternative is a bootable SD card with DISKBOOT.BIN, which loads automatically.
One thing STICK does need to work: it has to be able to read the firmware version off the camera. If it still won't cooperate after you know the version, download the build for that version manually from the wiki Downloads page and copy it to the card yourself.