Do you have the option of selecting a keyboard when the cursor is in an edit area? It should appear as a keyboard icon in the bottom-right of your device. Touching that icon should give you a keyboard selector. Editable fields of any application should support this unless you are on an explicitly constrained android release.
If you don't see the keyboard icon when you are editing text, it is possible that your device does not allow user provided keyboards.
If you do see the keyboard icon but JKeyboard is not an option, then you likely need to explicitly grant it permissions through settings/applications