Since Google's blogger parse each and every thing as an xml that is your blog is remained to a single xml file (for testing export your blog and see).That is why ad codes of various ad networks can not be placed with ease.For that purpose you have to convert that javascript ad code to xml compatible code.This xml parseable code is not ready to be embedded in any template.This problem basically arises in case of Adsense,adbrite,chitika and others

for converting ad code click here