I have a problem finding the "enter" I'm new to tablet asus infinity ken

The easiest way to find it is to open the internet browser and touch on the URL/Path line at the top, as if you were going to enter a URL manually.  Look for the "Go" button when the keyboard pops up, this is the ENTER key.  The name or symbol on the button will change (i.e. Go, Done, <--|), depending on the application you have open, but this will always be the ENTER button and it will always be in the same place.

There are different keyboard apps out there.  As you get more familiar with the tablet, you may want to look at Swiftkey 3, it supports both a standard keyboard mode and a split keyboard mode (default).