Hello Juan
Keep in mind that traffic from a lower security level to a higher security level is denied by default. In general, a DMZ will have a higher security level than the outside interface, so in order to go against this default behaviour, an access list which will permit such traffic must be applied.
Now the ACL itself is defined globally using the well-known access list syntax. Once it is defined, you must then apply it to an interface specifying an in our outbound direction. You can find out more information about how to apply access lists on an ASA at the following lesson:
For your specific question, you must create an ACL that permits the destination IP address and port of the server in the DMZ and apply it to the outside interface on an inbound direction. In the above lesson, Rene describes just such an example in the section titled Permit Traffic to DMZ.
I hope this has been helpful!
Laz