tweet
is not a string or a character buffer. It's an object. Replace your line with saveFile.write(tweet.text)
and you'll be good to go.
saveFile = open('MarSec.csv', 'a')
for tweet in MarSec:
print " "
print tweet.created_at, tweet.text, tweet.lang
saveFile.write("%s %s %s\n"%(tweet.created_at, tweet.lang, tweet.text))
saveFile.close()