.. and why do I get an error, that Concrete5 is looking for file view.php, when I try to view a Single Page I added?
First read a bit about Single Pages and their relation to view.php here.
So, when ever you add a Single Page yourself, or they are being added by a Package you installed, it will be looking for view.php in your theme directory.
What does it include? Just like your full.php or default.php files, you include some header/footer code, and a single line is needed.
<?php print $innerContent; ?>