Slight error in the previous version...
<?xml version="1.0"?>
<button backcol="none" display="both" separate="yes" textcol="none">
<label>Select every &nth file...</label>
<tip>Selects every nth file from the current lister</tip>
<icon1>#default:selectwild</icon1>
<function type="normal">
<instruction>SelectEx NTH={dlgchoose|Select Nth File:|2+3+4+5+6+7+8+9+10}</instruction>
</function>
</button>