To use the Java8 jdk which includes JavaFX in eclipse:
1. Add the Java 8 JRE to your eclipse workspace (once per workspace).
   a. In the menus, navigate to Window->Preferences.
   b. In the filter text box type: Installed JREs and click on "Installed JREs"
   c. Click the "Add.." button.
   d. Select "Standard VM" and click "Next >".
   e. In JRE Home type: /usr/java/java8
   f. In JRE Name type: Java 8
   g. Click "Finish"
   h. If this workspace is only being used for Java 8 applications, select the checkbox next to Java 8.
   i. Click "Apply and Close"
   
2. Create a project and specify it will use Java 8 (once per project).
   a. Create a project with either File->New->Java Project or File->New->Project:JavaFx->JavaFX Project.
   b. Click "Next".
   c. In the JRE section select "Use a project specific JRE".
   d. Select Java 8 from the dropdown next to "Use a project specific JRE".
   e. Continue creating the project as usual.