I have oracle forms bulider 6i and i installed oracle 10g express database. But i cant use forms 6i to connect myself with my database 10g.

Please check your TNSNames.ora file in NETWORK folder where you have installed forms6i.
You must have a TNS entry of the database which you are trying to connect.
The quick way is ; you can copy contents of TNSNAMES.ora file from your database into forms6i TNSNAMES.ora which you will also find in the NETWORK folder of your DB.

Check TNSNAMES.ORA file .

The TNSNAMES.ORA is a configuration file and the entry inside this file is used to connect to a oracle database server.
This file exist in oracle client pc and from the client computer this file entry is used to resolve
oracle service to establish connection to oracle server. It is nothing but contains net service names
mapped to connect descriptors for the local naming method, or net service names mapped to listener
protocol addresses. Note that we can easily connect to oracle database server without the help of
TNSNAMES.ORA.