First off make the height of the seat part of the chair 500mm from the ground in Sketchup also make sure the chair is facing the same way as the chair in the second picture below (In Sketchup), notice the arrows, now import your chair and change it from a prop to a seat then select bones, scroll down to just sitting and select it, now save and publish as normal if all goes to plan your characters should now be able to sit in your chair.
NOTE: Some times for what ever reason this work around does not work, but can start working if you redo the whole procedure again, well it has for me in the past.
(I also used a standard MS chair for this tutorial)
UPDATE INFORMATION FROM BEN_S
However, if you have a non-animated prop (such as any sketchup import), then you can change the skeleton without any ill effects.
It is thus possible to use a skeleton that was designed for one of the other chairs in your new prop.
This involves finding the props folder after you have imported it, in a file browser.
Look for the .csf file - that is the skeleton for your prop.
Delete that skeleton, and replace it with a skeleton from a working chair (eg the attached file - no need to change the filename after you have unziped it).
Go to the modders workshop and edit the prop type to Chair, and set the bone up for the sitting pose (as previously described). Republish.
Hopefully it should then work.
Edited by davidwww, 06 June 2009 - 12:45 AM.
turned inline images into links until we can fix the issue - Lisa