Ho preso il tuo codice e l'ho prima corretto:
codice:
<script type="text/javascript"> 
function enable_iscrizione() 
{ 
document.getElementById("activatebutton").style.display = 'block'; 
} 
</script> 

<form name="plus" action=""http://top.forumcommunity.net/" target="_blank" method="POST"> 
<input name="act" value="Poll" type="hidden"> 
<input name="t" value="24087160" type="hidden"> 
<input name="poll_vote" value="67" type="hidden"> 


<input type="image" onclick="enable_iscrizione(); return true;" src="http://i40.tinypic.com/21l8v7o.gif" class="forminput"> 


</form> 



<form action="/?act=Members&g=1" method="post"> 
<input name="s" value="c=19" type="hidden"> 
<input name="CODE" value="01" type="hidden"> 
<input type="image" src ="http://i40.tinypic.com/33kvtiq.jpg" class="codebuttons" type="submit" id="activatebutton" style="display:none;"> 
</form>
... quindi ho applicato le modifiche(in rosso le cancellazioni)
codice:
<script type="text/javascript"> 
function enable_iscrizione() 
{ 
document.getElementById("activatebutton").style.display = 'block';  
document.getElementById("primoForm").style.display = 'none'; 
document.getElementById("secondoForm").style.display = 'block';  
} 
</script> 

<form name="plus" action="http://top.forumcommunity.net/" target="_blank" method="POST" id="primoForm"> 
<input name="act" value="Poll" type="hidden"> 
<input name="t" value="24087160" type="hidden"> 
<input name="poll_vote" value="67" type="hidden"> 


<input type="image" onclick="enable_iscrizione(); return true;" src="http://i40.tinypic.com/21l8v7o.gif" class="forminput"> 


</form> 



<form action="/?act=Members&g=1" method="post" id="secondoForm" style="display:none;"> 
<input name="s" value="c=19" type="hidden"> 
<input name="CODE" value="01" type="hidden"> 
<input type="image" src="http://i40.tinypic.com/33kvtiq.jpg" class="codebuttons" id="activatebutton" style="display:none;" > 
</form>
ciao

ps: sei sicura che i tag <body> e </body> siano presenti? perche' senza non puo' funzionare...