If it's not showing up, that's likely a template issue that's hiding it in some way. I'd recommend using the browser's debug console to inspect the input fields and see if you can determine the cause. If not, please post a URL where we can see the SCLogin module being used and we'll try to help investigate further.
Could you tell us how you added the SCLogin module to your page? If I hover over the link to log in the module then shows. Normally, we have a modal popup option that is launched when a link is pressed. I'm wondering if this could be related. If you place the module on your page differently than this (not being popped up on hover), is the behavior the same that the placeholder is not shown?
i put the module at a different position, without hovering etc, it still doesn't show placeholders. It must be a template issue cause i checked it with protostar and it was working correctly. But i still can't figure out what excactly is causing the problem.
Sorry for the delay. Could you let me know a URL where I can see the SCLogin module without the hover behavior? We're having a hard time inspecting the elements on the page and getting the module to stay up to be inspected.
I'm unsure why the placeholder isn't showing in the module. I looked at the CSS and HTML and things look correct, but obviously something is preventing the placeholder from showing. Unfortunately, I don't know what it is. Looking at your page further, I see the search box at the top has a placeholder value set to "Αναζήτηση". That also is not showing, so it's not an issue with just our SCLogin module but something more with the site or template itself.
I'd recommend trying a different template to see if the placeholders work. If so, talk with your template vendor to see if they can help. If even the standard Joomla templates don't show the placeholder, let us know and we can investigate further on one of the standard Joomla templates which we know better.