Please specify which CS5 product do you have?
Well, since CS5 is not part of the Cloud, you may also want to check in the forum for your program (start at the link below)
I have CS5 Master Collection... and when I bought the discs I had to specify Windows or Mac... so I don't think you may install on both
If you start at the Forums Index http://forums.adobe.com/index.jspa
You will be able to select a forum for the specific Adobe product(s) you use
Click the "down arrow" symbol on the right (where it says ALL FORUMS) to open the drop down list and scroll
Can the same software package CS5 be loaded on a mac computer and also a PC?
The answer is definitely no.
Creative Suite serial numbers are platform specific: Windows only or Mac only.
To swap from one to the other you'll have to upgrade to CS6 or join the Cloud. Cloud software can be used on Mac and/or PC.