Here's my template which is not incrementing the
count value whenever <mo>(</mo> is matched in the
document.
<xsl:template match ="mml:mo[.='(']">
<xsl:variable name ="moWithParen"
select="count(preceding::mo[text()='('])"/>

I guess you forgot the namespace here. Try
 select="count(preceding::mml:mo[text()='('])"

<xsl:text>&lt;form name = "</xsl:text><xsl:value-of
select ="$moWithParen"/><xsl:text>"&gt;</xsl:text>