SQL & PL/SQL :: How To Connect User From Another User

I like to know how to insert/select a table from a user to other user.

e.g. :

if i give this below command the data is not inserting.

INSERT INTO MYDATABASE.MYTABLE Select * from MYTABLE where YEAR = '2011'

OR

SELECT * FROM MYTABLE@MYDATABASE WHERE YEAR = '2011'

if i execute the above commands i get the error: Table or view does not exist

Connection to remote database not found

I have made a database link to that database but its not working.