First think of the multiplication table from grade school and then check out the following link: http://www.biomedcentral.com/content/supplementary/1471-2164-9-617-S5.pdf
The only thing different about the percent identity matrix from Clustal Omega is the missing top row containing the sequence descriptions. So to make comparisons in Clustal Omega just pretend the top row containing the names of the sequences is available.