if its wallet client is based on the coding originally based on the bitcoin-qt client
Help -> debug window -> console
then type in:
importprivkey 1T3RrAc01nAddr355blahblah
where you change 1T3RrAc01nAddr355blahblah to your actual PRIVATE KEY
but it may be best that you use
http://www.terracointalk.org/ for all future terracoin chit chat, or the alt-coins sub section