Screenshots shown are from Apple Mail in OS X Yosemite (10.10).
Open Mail. If you already have an account setup in Mail then go the Main menu, select Preferences, click Accounts and then click the plus sign (+). If you have not setup an account in Apple Mail then it should open directly to the new account screen.
Select Exchange from the list of account types, then click Continue.
Enter your name, e-mail address and Trojan Web Express password, then click Continue.
A successful autodiscovery should result in a screen similar to the one below. Note that this information may not be displayed in different versions of OS X. Click Continue to proceed.
Features such as contacts and calendars can also be used with Office 365. Select the options you want to use, then click Continue.