You shouldn't need to install anything. Just in case you are doing something different to open the help, here's how ...

There are TWO ways to open the Poser manual properly.

1.  From the Poser application, choose Help > Poser Manual. That will open the Poser Documentation home page in your default browser.

or

2.  Go to www.posersoftware.com. Click the Documentation link at the top of the page and choose Poser 13 Manual HTML.

If, after doing that, your browser doesn't look like the following screen shot, try clearing your cache again. 

PS the copyright notice that says 2020-2023 needs to be updated so don't let that throw you off about the update right now. There's a snippet that I haven't caught in the docs yet it's on the to do list. 8-)