this errror is commonly caused by missing library, have you imported in your project this:
import com.google.gdata.data.media.mediarss.MediaThumbnail;
?
if not first import in your android project library by download it first:
download-link: https://code.google.com/p/gdata-java-client/downloads/detail?name=gdata-src.java-1.47.1.zip&can=1&q=gdata