After importing your sequence of interest, simply click and drag within the Sequence Map to highlight the region you wish to BLAST. You can also use Command+A or Ctrl+A to select the entire sequence. Keep in mind that BLAST is limited to 5000 bp.
Click the BLAST button above your sequence map. You'll automatically get routed to NCBI BLAST.
You can also right click on your sequence, and select NCBI BLAST from the drop down menu. Just one more way to get there!