Launch OWB and open your project
Expand the Database node
Expand the target module
Right click on Materialized Views and select Create
A wizard pops up containing 6 steps
Give it a name
Define the name, data type, and whether mandatory for each column
Type or paste the MV SQL - just the select portion
Define constraints - optional
Define attribute sets - optional
Finish
After the MV has been created you can get back to the above wizard using right click on the MV and from the pop-up selecting Properties
To set Build and Refresh characteristics, right click on the MV and from the pop-up select Configure