AppleScript text strings are enclosed in quotes, and the concatenation character is the ampersand &. In addition, quotes need to be escaped with the backslash character (you can also use the string constant quote, which doesn't need to be escaped), and of course a backslash character also needs to be escaped.
If I understood your snippet, it would be something like:
set theText to "<title>" & ArticleTitle & "</title>" & return & "<dc:creator>admin</dc:creator>" & return & "<content:encoded><![CDATA[" & ArticleSource & return & return & ArticleDescription & "<a href=\"" & ArticleURL & "\">More</a> & </content:encoded>" & return & "<excerpt:encoded><![CDATA]></excerpt:encoded>" & return & "<wp:post_date>" & ArticleDate & "</wp:post_date>"