I am not sure what the problem is. Some simple things to check (you are probably doing all of these things already, but I have to be sure):
1. Make sure you are using the system clipboard when copy/paste between applications. This means using the middle mouse button to paste, and select-and-leave-selected to get a selection onto the system clipboard.
2. Make sure that MButtonDn (the middle mouse button) is bound to mou_paste within SlickEdit (Tools>Options>Key Bindings, Search by key sequence).
3. Make sure "Mouse selection creates clipboard" is ON (Tools>Options>General, Selections).
If none of that sheds any light on the problem, then send email@example.com
an email and reference this forum thread.