You can put buttons into the header, but not the body. If you want to put buttons into the body, you should use a list view layout instead.
To make the header visible in a table view, enter layout mode, select layout setup, click the views tab and then click the properties button associated with table view.
Well, indeed this is a much needed feature but you can workaround it with script triggers :
Just put a on object enter script trigger on the filed you want a button on, make sure you put a commit script step at the end of the script if you want your field to remain uneditable.
Of course it has the drawback that this locks the record temporarely, but most of time it's not a conern.
BUT I REALLY WISH FMP TO GET THIS FEATURE