(Since this question was posted 3 years ago, I'm not sure I should be answering, but forgive me for being a noob)
I think there are a few things you want to look at:
First - Reusable Workflows typically want to use "Site Columns" that are associated through the "Association Columns" button. I think the reason for this is to help ensure that your workflow can find the column no matter what list you attach it to. So, rather than making your new field in the list, you want to make it in the "Site Columns" part of SharePoint Designer. After creating the Site Column, go into the Workflow and click "Association Columns", then click "Select Site Column" - you should now see your new column. When you add it, you will now have it available to use in your workflow. Also note that this new column will get attached to any task list that you attach the workflow to, so you don't need to manually create the column in each task.
Secondly - I've had some issues attaching reusable workflows onto task lists. I'm no expect, but I've found it useful to select "Task" list as the template when creating both the workflow and the List. If you create the list from scratch or using a different template, then you may not be able to attach a workflow designated for a Task list later on. If both your workflow and list were created using the "Task" template, then your list should appear in the "Associate To List" option when viewing the workflow in SPD. If you get an error, remember to republish the workflow (sometimes I need to do this multiple times to avoid getting an error). Also, a final note that once you choose "Associate To List", SPD will send you back into your SP list and ask you to configure some parameters, such as a unique name for the workflow - I find this page does work and often have to re-select "Task" list template in order for it to work.
Again, sorry, I know answering questions after 3 years is not likely to be helpful - but I figured someone else may come along and appreciate seeing an answer.