Pa ako tražiš referencu na formular u događaju formulara, onda je to this...
Kôd:
this.elements ['data[percent]'].value
ili kompletno
Kôd:
<form name="ime" action="javascript: alert ('wow');" method="post" onsubmit="return confirm('Are you sure that interest for this month is '+ this.elements ['data[percent]'].value +'%');">
<input type="text" name="data[percent]" value="" />
<input type="submit" value="Distribute" />
</form>