If you had previously installed the plugin without the translation files in place, you will need to force the translated field names into the database. See this post for explanation and instructions:
http://butlerblog.com/2011/07/13/forcing-translated-field-names-when-using-translation-files-with-wp-members/
German translation for fields and buttons do not seem to work for this on my site. Do I have to install any mo.or po. stuff in addition to just having a german WP-install (version 3.3.2–de_DE)?
Thanks in advance.
@win7mac – for any plugin to be translated, you need to have (1) the correct .mo file for a plugin that is localized and (2) the WP installation needs to be running the same language.
There is a German translation for WP-Members available here: http://butlerblog.com/wp-members/wp-members-translations/
If you have previously installed the plugin without having the translation file present, there are some elements that you may need to force into the database. See this post: http://butlerblog.com/2011/07/13/forcing-translated-field-names-when-using-translation-files-with-wp-members/
I should have also mentioned with the above explanation that for WP-Members, you need to load the .mo file to the /lang/ folder of the plugin.