First make certain that you are using the clipboard to do a copy and paste of the key (rather than trying to type the key into the text field manually).
If ServletExec is still rejecting your key with a message such as:
! INVALID LICENSE KEY -- PLEASE TRY AGAIN.
Then please make certain that there is no leading or trailing whitespace in the key that you are trying to feed to ServletExec.
A license key for SE X.something will work with SE X.anything.
In other words... license keys are "tied" to the major version of SE, and will work with all point releases of that major version.
For example an SE 4.1.1 key will work with SE 4.1.1 and SE 4.2.
An SE 5.0 key will work with SE 5.anything, but not with SE 6.0.
In the email that was sent to you containing your purchased license key(s), the version for which they were generated should be specified. Make sure that you are registering the correct version of ServletExec for the version of key that you have.