stkx files are same as stk, but locked and need the purchased version of the above application to open.
if you find any other way to open stkx please post back.
the fix for this is rather simple first however, you need a really good text editor (notepad in windows will crash if you try this with it) i suggest using Notepad++ (http://notepad-plus.sourceforge.net/uk/site.htm).
with an appropriate text editor, open the .stkx file with the text editor
you'll see a bunch of garbled text, but do not fret. if you scroll over to the right a bit, you'll see some legible english within the first two lines.
it should look something like this: obj<</Type /Catalog /Pages 3 0 R /Cover 2 0 R /EncryptType /Key /ReadDirect /Right /Comic /Yes /FileID /21AAD177B1A40E4E95231268635BFC01>> endobj
the place of real importance is where it says "/EncryptType /Key" if you just change the "/Key" to "/NoKey" (exact capitalization), it'll work perfectly like any normal .stk file with eRead