A list of attributes available for the selection in M2E Pro includes only Text Field / Price Type attributes which are presented in all Magento Attribute Sets.
If you need a specific Magento Attribute in the Module, you should add it to all your Attribute Sets in Magento. To do it, please navigate to Catalog > Attributes > Manage Attribute Sets in Magento v1.x and Stores > Attributes > Attribute Sets in Magento v 2.x.
Tip
Starting from M2E Pro v.6.5.8, Magento Attribute dropdowns show all attributes. So Magento v.1.x users can upgrade their M2E Pro instance to overcome the inconvenience.