Do you simply want to plot the individual expression values for a particular gene? Most people use cpm() or rpkm() to extract individual expression values from the DGEList object. Then you can use standard R plots to depict the values for any single gene.

As you will know from reading the documentation, you need to use topTags() or decideTests() to assess which genes are significantly differentially expressed. Obviously you can't just judge DE from a smearplot or MD plot, because those plots only show fold changes without showing variability.