Italy02,
If you have rebooted as scorpio-logic indicates, and then clicked a skin in WindowBlinds configuration panel to show the skin in the preview pane - you should be able to do one of the following to apply:
1 - Click the "Apply this skin now" link (found inside the skin preview pane)
2 - Click the "Apply changes" link (found on the left side of the configuration panel)
If neither of these work, please use the "Troubleshooting" link to generate a troubleshooting report, and then copy and paste the text of the report into a new comment in this (or a new) thread.
Someone from Stardock should be dropping by to check on posts, and to see if there are any bugs that have not been addressed and fixed.
Note - if you are not able to apply a skin after trying the steps above, I would suggest trying to uninstall > reboot > reinstall > reboot - to make sure that the activation and registration of program components is completed successfully. Since it is a beta, there may have been a simple glitch during installation, or there may be an issue with compatibility.
Note #2 - if all else fails, you can uninstall > reboot > start Stardock Central, and deselect the "Show pre-release versions" > install WB 4.6 until a fix comes about.