Here is the direct link to the Mexico On line Immigration and Customs forms.
Note: You can't use this form more that 3 days in advance of your arrival date
https://siat.sat.gob.mx/PTSC/DPE/faces/resources/pages/declaracion/declaracion.jsf
On the upper right of the page click English to change the forms to that language then fill out the forms and follow the instructions
FYI
- The Mexican State your are going to is: Quintana Roo
- The nearest city/town is: Puerto Morelos
- The address you are going to - just enter the name of the resort: Desire Riviera Maya or Desire Pearl