If a supplier isn't assigned to a project, it won't appear in the Fee/Budget Plan. To assign a supplier to a project, go to the project, click on Fee/Budget Plan and then select the Sub-Consultants Plan.
Click on the supplier console button (its a blue building icon) that's just under the name of the project and this should open a pop up window with a list of supplier accounts.
Find the supplier you would like to add to the project from the left hand side, and then click on the arrow icon pointing towards the right hand side, to add it to the project.
Click on the save icon at the bottom of the screen and this should add the supplier onto your project. 