Current mana level is EQType 124.
This is the code taken from my modded UI
<Label item="MNANumberHB">
<ScreenID>MNANumberHBLabel</ScreenID>
<EQType>124</EQType>
<Font>1</Font>
<RelativePosition>true</RelativePosition>
<Location>
<X>106</X>
<Y>168</Y>
</Location>
<Size>
<CX>25</CX>
<CY>15</CY>
</Size>
<TextColor>
<R>52</R>
<G>159</G>
<B>250</B>
</TextColor>
<Text>7000</Text>
<NoWrap>true</NoWrap>
<AlignCenter>false</AlignCenter>
<AlignRight>true</AlignRight>
</Label>
Font, and text color can change based on your preferences. Location and size will depend on where you want it in whatever window you put it in.
You will also need to add it to the Screen block under pieces.