In Serpent’s Bluff Redoubt – Page 534 of the Guide it says to open this particular door (a sort of grill) place a small object on the pressure pad on the altar. I don't get a message to that effect in the game that I could click on.
I tried dropping and that didn’t work and I can’t think of anything else.
How do you place things?

If you hold down the "use/pick up" button when the crosshair is over an object, you can get it into a "held" state which allows you to move it around freely. The object will drop when you tap the use/pickup button again or if the object gets bumped away from the player with enough velocity. This applies to objects you drop from your inventory as well as (most) objects found around the environment.

It can be tricky to tell that you are now "holding" the object, so it takes a bit of getting used to, but then it's pretty simple. You can also use FUS to move an object onto the switch, or walk into a loose kettle or something similar to punt it onto the switch. You can even drag an enemy corpse onto a pressure switch, but this is pretty cumbersome. Note that you can't hold some random objects like animal skulls but you can knock them around in the other ways mentioned.

Hope this helps!

Thank you very much. This was a geat help. I'm off to do it now.
Happy New Year and thank you again

I ran into a problem. What is the use/pick up button?
I've looked in the booklet where it sets out the gameplay controls and it doesn't tell me.
I'm playing on a PC if that makes a difference.

I'm on xbox, and here it's the 'activate' button, with which you travel doors, talk, etc... Maybe this applies to PC as well?

Should be E, unless you've remapped it. Otherwise, whatever button you use to pick items up, hold it down longer. The "Take" will disappear and if you move around you will drag the item.

Yes, it was the E and it worked.
Thank you both for you help