Plugins menu doesn't appear in the admin panel

The issue is almost certainly related to one of two things:

1) You don't have permissions (are you logged in as admin?)

OR

2) The theme (or a plugin) has disabled the plugins menu.

After you've checked to be sure that you are an administrator, then you should attempt to view this page:

http://www.yourdomain.com/wp-admin/plugins.php (replacing www.yourdomain.com with your site's domain name).

If it's accessible, then it's time to start exploring the theme (which theme are you using?) and possibly some of the plugins that might have disabled the menu item.

FYI, the method to remove dashboard menu items is covered here: http://www.wprecipes.com/how-to-remove-menus-in-wordpress-dashboard

If you did double check your file permissions (i.e. 755 for folders and 644 for wordpress files on Linux), please consider editing your wp-config.php file and change

define('DISALLOW_FILE_MODS',true);

to

define('DISALLOW_FILE_MODS',false);

The Add New option in Plugins menu will show up, also if you have internet connection, possible updates will be shown up.

This what I found looking for the same problem, I guess this is your situation also:

If your site is hosted on WordPress.com:

Plugins: Plugins are tools used to extend the functionality of the WordPress platform. However, they are only applicable to self-hosted blogs and web sites using the WordPress.org software. Plugins are not permitted here at WordPress.com for various security reasons.