There´s flights from Tobago to Margaritas, but I can´t remember their name eighter.
But if you are in Tobago, go to Caribbean Holidays nearby the Airport. Their office is situated on the road from the Airport to Store Bay.
That company is dealing with visitors, tours and so forth, and I know that some of their guests sometimes goes to Margaritas.
If you aren´t in Tobago, here´s some addresses
http://www.caribbean.as
Phone +1-868-631-2000
Fax +1-868-631-5011
E-mail
tore.svalesen@caribbean.as
Good luck
Ronald