How to send an email with embedded values
You have to use dbname.schema.objectName e.g. dbname.dbo.MYTABLE in your tsql

EXEC msdb.dbo.sp_send_dbmail
        @recipients='MYMAIL',
        @body='100 Daily documents downloaded', 
        @subject ='100 Daily documents downloaded from user',
        @query = 'SELECT USERID FROM dbName.schema.MYTABLE WHERE MYCOUNTER &gt; ''100'''


EDIT:

You only want to send email when there are 100 Daily documents downloaded from user, so you can add

if exists (
        select 1
        from dbName.schema.MYTABLE
        where MYCOUNTER &gt; '100'
        )
begin
    exec msdb.dbo.sp_send_dbmail @recipients = 'MYMAIL'
        ,@body = '100 Daily documents downloaded'
        ,@subject = '100 Daily documents downloaded from user'
        ,@query = 'SELECT USERID FROM dbName.schema.MYTABLE WHERE MYCOUNTER &gt; ''100'''
end
else
    print ' the user has not downloaded 100 docs yet'


Note: Why are you storing integers as chars ? e.g. 100 is stored as '100' ?