I've seen the next/back button overlap as well, but never so much that you can't click the correct one.
After you select CSV and click Next, it takes you to the second step. At the top of the screen you'll see a wizard bar with the steps, and the first one (the highlighted one) is "Paste".
A short bit of history/reasoning: Because of the way web browser security has been structure/enforced, to select a file would require sending the file to the server (perhaps over SSL, but in such a way that the server could read it). Clipperz is a zero-knowledge system, where the server never receives data from you that it can understand (everything is encrypted), therefore using the normal "Select File" mechanism of the browser would be bad for security. Instead you should...
Open your CSV file in a text editor (Notepad, TextEdit, whatever you have handy)
Highlight all (with Ctrl-A or Command-A)
Copy
Paste into the text box in Clipperz
Click the "Next" button and it should take you to column selection
I hope this helps!