If I program a website and use a JS plugin (http://responsiveslides.com/ for example), which is licensed under MIT. Does this mean I have to put the MIT-license somewhere in my directory. If so, where do I put it and does the user of the website have to be able to see it somehow?
What is not clear in this?
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
So yes, you have to.
The best place to put the license is, I would say, in a comment in the code of the JS plugin itself. It is short enough so that it won't be a problem.
If you minify the source before distributing it, you should make sure that the minified version still contains the license (preferably at the top, with some explanation on what it applies to). So yes, the user will be able to see it, by dispaying the source of the page.
I don't think anything forces you to have an About page to acknowledge the use of this library, although this is nice to do so.