Apps
Depends upon how the app is being marked as incompatible.
If it's marked by myself as incompatible, then the following will work:
Apps, Settings - Enable Show Incompatible Apps
If it's marked by the author itself as being incompatible, then you have to manually edit the .plg and remove the constraints (eg: Max in the .plg entities) and then once saved onto your flash drive somewhere, install the locally saved version. IE: This is a pain to do, and if you're not following what I'm basically saying of how to accomplish in this circumstance then its a safe assumption that you shouldn't be doing this in the first place.
Either way, incompatibilities are usually set for good reasons and aren't recommended to be overridden