Q: I cannot download from the mac app store

Apple did not make the Mac App Store available until 10.6.6, and most Apps on there are only available for 10.6.6 and higher.  If you are really running 10.5.5, you should seriously consider getting 10.6 and updating to 10.6.8 because of a first virus to ever venture in Mac OS X, which affects anyone browsing with Java. Those with less than 10.6.8 can turn off Java to avoid it, but of course this means many websites won't be accessible. This tip explains how to turn off Java with 10.6.7 or earlier, or update to 10.6.8 and apply the Java patch:

https://discussions.apple.com/docs/DOC-3261

10.6 is still available from many places, but is not necessary for upgrading to 10.7.

Here are your options for 10.6:

https://discussions.apple.com/docs/DOC-2455

Here are your options for 10.7:

https://discussions.apple.com/docs/DOC-2465

References where to download 10.5 compatible software are here:

https://discussions.apple.com/docs/DOC-2275

http://support.apple.com/kb/TS3624