You do NOT have to install fonts in order to make them appear in PSP.
I have a folder in My PSP files that is labelled Fonts. All I do is open the font I want to use and it becomes available in PSP. You do not want to install too many fonts in your C:/Windows/Fonts folder as it will bog your system down. You can also download a font viewer from the web(there are several free ones) that, when you have it up and running, it will cause those fonts to be available in PSP as well.
For your brushes, more than likely you have a file type that is unsupported by PSP, so create a folder in your My PSP files labled Brushes To Import and save all those brushes in there (they are probably jbr types). Then you go to File/Import/Custom Brush, navigate to your brushes to import folder and choose the ones you want to import. The program will do the rest.
Once you have them imported, do go and delete those files out of your brushes to import folder otherwise the next time you need to import some, you will get an error that the brush already exists.
Margaret