Ok the multipolygon relation is called “Posto 9” and it’s is here
http://www.openstreetmap.org/#map=17/40.29684/17.53960
Josm complains that it has no outer path and that it’s empty
To me it’s not empty and it has indeed an outer boundary.
I don’t understand.
Any hint appreciated.
You misunderstood multipolgons. They’re for an area with holes, not for describing which elements are in another element. JOSM complains that it can’t create a valid geometry out of an outer-area and a few ways and some nodes as inner.
Sorry for this short explaination, propably someone else will make something more newbie-friendly out of this 
I’m not sure what you are trying to accomplish, but maybe the site-relation is what you are looking for: http://wiki.openstreetmap.org/wiki/Relation:site
If it is just a simple area, then no need for a relation.
Just map the outline as an area (closed way), then tag that. eg tag as leisure-beach_resort. It is obvious from the geometry that the cafe and toilets are inside the area, so are part of the resort.
Vclaw is right, if it’s a closed aread, just draw the area.
If the site consists of multiple non-connected areas, you can use the site-relation
Thank you people.
That’s right, it’s a closed area, everything is inside.
Thanks again