In a previous paper (J. Phys. Chem. A 2009, 113, 9721), we analyzed theoretically the Diels-Alder cycloaddition between cyclopentadiene and C(60) for which experimental results on energy barriers and reaction energies are known. One of the main conclusions reached was that the two-layered ONIOM2(B3LYP/6-31G(d):SVWN/STO-3G) method provides results very close to the full B3LYP/6-31G(d) ones. Unfortunately, however, both the exothermicity of the reaction and the energy barrier were clearly overestimated by these two methods. In the present work, we analyze the effect of the inclusion of Grimme's dispersion corrections in the energy profile of this reaction. Our results show that these corrections are essential to get results close to the experimental values. In addition, we have performed calculations both with and without dispersion corrections for the Diels-Alder reaction of C(60) and several dienes and for the Diels-Alder cycloaddition of a (5,5) single-walled carbon nanotube and 1,3-cis-butadiene. The results obtained indicate that inclusion, of dispersion corrections is compulsory for the study of the chemical reactivity of fullerenes and nanotubes.