If it's a ZIP folder, or a RAR file, you can open it with http://www.win-rar.com/download.html?&L=0
After you open it, locate the folder that has the PaintTool SAI.exe and its own folders in it.
Move the files from the brush and texture pack into the PaintTool SAI folders. (EX: brushtex files go into SAI's brushtex folder, elemap goes into elemap, papertex goes into papertex, ect!)
If you downloaded the brush and texture pack from DeviantART, make sure to read the description! It'll give any details you need, most of the time, including a how-to-install guide. (I hate it when people don't read the descriptions I write! XD)
Good luck!
I'm the worst possible person to explain things like this, so this might help! http://www.youtube.com/watch?v=MkQ7EElYKPc