Linktocategory not included?

You don't forget anything. linktocontent_category doesn't need a tinymce-plugin because it extends linktocontent_node (linktonode).
But the error message shouldn't appear because the specific function looks like this:

function linktocontent_category_check_files() {
  return true;
}

If it returns false, the message appears.
I've checked the archive and tried the files on a new installation: everything worked fine.
Which php-version do you use?
Can you check the result of linktocontent_category_check_files().
Insert the following code in linktocontent.module, line 109:

error_log(print_r($plugin_name, 1));
error_log(var_export(module_invoke($plugin_name, 'check_files'), 1));

and check your error log.
It should return

linktocontent_node 
true
linktocontent_category true

Hi.
The entres for linktonode and linktomenu should look like this (in plugin_reg.php):

// linktocontent.module: linktonode
if (is_dir(drupal_get_path('module', 'tinymce') . '/tinymce/jscripts/tiny_mce/plugins/linktonode/')) {
  $plugins['linktonode'] = array();
  $plugins['linktonode']['theme_advanced_buttons3'] = array('linktonode');
}

// linktocontent.module: linktomenu
if (is_dir(drupal_get_path('module', 'tinymce') . '/tinymce/jscripts/tiny_mce/plugins/linktomenu/')) {
  $plugins['linktomenu'] = array();
  $plugins['linktomenu']['theme_advanced_buttons3'] = array('linktomenu');
}

With this code in plugin_reg.php the buttons shall appear in the profiles settings.

editor_plugin_src.js isn't needed because its just the uncompressed source file of the plugins.

Ha, got it:
Your plugin_reg.php:

return $plugins;
}
// linktocontent.module: linktonode
if (is_dir(drupal_get_path('module', 'tinymce') . '/tinymce/jscripts/tiny_mce/plugins/linktonode/')) { 
	$plugins['linktonode'] = array();
	$plugins['linktonode']['theme_advanced_buttons3'] = array('linktonode');
}

// linktocontent.module: linktomenu
if (is_dir(drupal_get_path('module', 'tinymce') . '/tinymce/jscripts/tiny_mce/plugins/linktomenu/')) {
	$plugins['linktomenu'] = array();
	$plugins['linktomenu']['theme_advanced_buttons3'] = array('linktomenu');
}

The plugin definitions have to be a few lines above:

  // linktocontent.module: linktonode
  if (is_dir(drupal_get_path('module', 'tinymce') . '/tinymce/jscripts/tiny_mce/plugins/linktonode/')) { 
    $plugins['linktonode'] = array();
    $plugins['linktonode']['theme_advanced_buttons3'] = array('linktonode');
  }

  // linktocontent.module: linktomenu
  if (is_dir(drupal_get_path('module', 'tinymce') . '/tinymce/jscripts/tiny_mce/plugins/linktomenu/')) {
    $plugins['linktomenu'] = array();
    $plugins['linktomenu']['theme_advanced_buttons3'] = array('linktomenu');
  }

return $plugins;
}

Otherwise they aren't attached to $plugins and don't appear in "Buttons and Plugins".
Hope this was the solution :-)

greetings,

Stefan

If it would not be too presumptuous on my part, I did a little editing on the Install.txt to clear up where I (and maybe others) got confused.

Thank you! I will check this into cvs.

greetings,

Stefan

forget to set it to "fixed"