See this old blog post that has some examples for those settings
Custom Brand is just a brand name, you can also add a logo and control background and foreground colors from the blog settings.
You should not need to use a custom script from addthis.com since it is built in to mojoPortal.
The settings in layout.master are for when using the addthis button on every page, but the blog has separate settings for addthis.
Hope it helps,
ps this forum is for questions about working with source code in Visual Studio. this question should have been posted in the Questions About Site Administration or general Questions forum not the Developer forum