There are a two ways,
First Way
1. Go to the slide that you want to link to a website
2. type the web address into a text box
3. Web address should turn blue with an underline. (if not retype address and try again)
4. Go to slide show View
5. Click on the address (Should go directly to the website you wanted)
OR
Second Way
1. Go to the slide that you want to link to a website
2. "Right click" on a picture or text box on that slide.
3. Select "Hyperlink".
4. Then click on the "Browsed Pages" tab
5. Beside "address" type in or cut and paste the website address you want to link to.
6. Go to slide show view and test it.
I personally like the second way better, i just think that it has a better appearance during the presentation. Good luck.