Defaults and thumbnails

Hi,
really like this plugin.
There are 2 things which I cant fix yet.
The first is the thumbnail issue, read a few post already and tried to add add_theme_support('post-thumbnails'); to my theme’s functions.php.
Really didnt know how to do that but I think I found the correct .php and just pasted the code on the bottom. Still I cant see thumbnails. I’m doing something wrong I guess.
Almost all other plugins are disabled.
Perhaps someone can explain this tric in dummy style?

The second question is the defaults in the Add new listing fields.
Can I change the default values of Conditions from NEW to USED, and at the Transmission field from AUTOMATIC to MANUAL?
This will really be user-friendly for my client.