These are described in the solution to this issue: https://opensource.ncsa.illinois.edu/jira/browse/DFDL-81. See the comments at the bottom for the settings.
Intellij comes ready-equipped to support Daffodil with only a few changed required to add the plugins for Scala and SBT support. In the Intellij UI, open File->Settings and in the left pane scroll down to the section labeled Plugins (this is below the middle divider). In plugins select "Browse repositories,,," and from this list select "Scala" and "SBT" (use the search feature to help find them). Then click the Download and install icon (the disk with the arrow toolbar button or via context menu). When the two plugins are loaded, close Intellij and restart.
Once restarted, reopen settings and open the SBT option under the project settings (you can ignore the Scala settings as these defaults are sufficient). Under "SBT launcher JAR file" enter the path to where you downloaded the sbt-launch,jar above and under the VM parameters enter the value specified by the JAVA_OPTS export.
Under the Edit Configurations menu (drop-down) make sure you have a target. You can create a new target to build all tests by clicking the plus sign and then selecting "JUnit" as the test type. Then in the dialog make sure you select the correct class path (i.e. daffodil-core) and path to the core and then check "Run SBT Action" and in the "..." dialog select "test:products".