ok for anyone else interested i got it working. my 2.5.7 site was originaly a 1.7 site so I think this had somthing to do with it.
Im not sure which step did it but here is what I did.
first in extension manager/discover/
click Purge Cache - then click Discover - select all and click install
You will see all the latest joomla plugins that were not included in 1.7
Go to plugin manager and enable all the plugins
Then click warnings again in extension manager. If your getting a temporary php folder error like this
The PHP temporary directory is not set
The PHP temporary directory is the directory that PHP uses to store an uploaded file before Joomla! can access this file. Whilst the directory not being set isn't always a problem, if you are having issues with manifest files not being detected or uploaded files not being detected,
you may need to create a php.ini file in your administrator folder and put this code in it
upload_tmp_dir = /home/YOURCPANELUSERNAME/tmp
Change YOURCPANELUSERNAME with your own cpanel username
also the second line "upload_max_filesize=8M" I added because of the next error I got when I fixed the first error. You may not need to do it.
More info on inmotion support page http://www.inmotionhosting.com/support/edu/joomla-25/305-php-temporary-directory-not-set
Then go system maintenance and do all the checkins and clean cache
Then jcomments still not working, don't bother trying to re-install over your current install it still wont work. FIRST uninstall jcomments and then re-install.
Now you should have a working jcomments component on your joomla 2.5.7 site upgraded from 1.7
hope this helps