Strictly speaking there are not really any "pre-view" but a "post-view". Probably this leads to think that there are something wrong in your installation, but probably is working fine.
When you start
texworks empty or with a new file, said
somefile.tex file, there are no any "preview" window simply because still there are nothing rendered to show.
But once you compile successfully that new file (green upper-left buttom) with the right compiler (after selecting
pdfLaTeX or whatevercompatible with that file) and the
somefile.pdf is created, then the "preview" must appear to the right of the editor window.
If you close
texworks and run again
texworks somefile.tex now the preview window is opened immediately at the start since the PDF
somefile.pdf already exist in the same directory.
If you close the preview window but not the editor you can open again the preview with a new compilation or (without compiling again) in the menu
Window > Preview. If the preview is open but hidden by the editor window or another program, this or
Window > somefile.pdf also will put the focus in the preview window. Of course, this cannot work if you move/delete the PDF file meanwhile.