Change
$+(%seen.,$did($seen,1).seltext)
to
$($+(%,seen.,$did($seen,1).seltext),2)
You need to evaluate the variable. Notice the comma (,) between "%" & "seen.". It needs to be separated so when mIRC evaluates it, it evaluates the WHOLE combined variable, and not just %seen. which has no value & which is why NICK is only showing.
** $() is short for $eval()