I recommend reaching Tour Saint-Jacques by mass transportation. It is way cheaper and most of the time much faster than the alternative taxi.

Coming by plane:

From Charles-de-Gaulle airport (CDG), take RER B and exit at station Les Halles. The train ride lasts around 32 minutes. It is a direct train.

From Orly (ORY), take the Orlyval which connects with RER B at Antony. Exit at station Les Halles. The journey will take about 38 minutes.

Coming by train:

From gare du Nord, gare de l'Est and gare Montparnasse, take metro line 4. It is rapid but beware that you will hate the long staircases, having a suitcase in hand! Stop at Châtelet. Choose the exit labelled "place Sainte-Opportune". That's the best.
Climb the stairs to the street level, then make a U-turn staying on the same sidewalk. You arrive on rue de Rivoli, you cross rue Saint-Denis and then you cross boulevard Sébastopol. You keep going a few meters, you pass by a coffee shop, a parapharmacy, a clothe shop "un jour ailleurs" and here you are, ready to ring under my name.

From gare de Lyon, you may take either metro line 1 or metro line 14. Choose the later, there are more mechanic staircases, and it has less stops. Exit at Châtelet.

The last meters to Tour Saint-Jacques:

I recommend you print out the google map of the neighborhood to guide you through the last meters from mass transportation to Tour Saint-Jacques.