I would suggest having a look at
http://docs.quantumwise.com/tutorials/builder_manual/builder_manual.html, especially, reading the section on 'Move Tool'.
As a general guideline:
1. Add PtO2 crystal from the Database to the Stash.
2. Convert it from bulk to molecular configuration, using the corresponding plugin in the Builder.
3. Select and delete all the atoms except the O-Pt-O atoms aligned in a row. This is how you get your O-Pt-O molecule.
4. Rotate the row of atoms such that it is aligned along the z-axis, using the Rotate plugin in the Coordinate Tools.
5. Click on the actual graphene structure in the Stash.
6. Drag-and-drop the O-Pt-O molecule from the Stash to this structure to merge the molecule and graphene structures.
7. Select the O-Pt-O atoms.
8. Use Move Tool to position the molecule where you want on the graphene sheet.
The detail information on the Builder plugins is given in the tutorial I have mentioned.