FCLoopAddEdgeTags[edges_List, labels_List] adds user-defined styles and labels to the given edges using the provided list of labels. Styles and labels are attached using the replacement rules provided via the Style and Labeled options.
If you use FCLoopIntegralToGraph for visualizing loop integrals, the first two entries of its output can be used as the input for FCLoopAddEdgeTags, e.g.
FCLoopAddEdgeTags[FCLoopIntegralToGraph[FAD[p, p - k], {p}][[1 ;; 2]]]
GraphPlot[%]\{-3\leftrightarrow 2,-1\leftrightarrow 1,1\leftrightarrow 2,1\leftrightarrow 2\}
If you just want to plot the obtained graph, it is easier to process the output of FCLoopIntegralToGraph directly with FCLoopGraphPlot, which internally uses FCLoopAddEdgeTags.
FCLoopIntegralToGraph[FAD[p, p - k], {p}]
FCLoopGraphPlot[%]\left\{\{-3\to 2,-1\to 1,1\to 2,1\to 2\},\{k,k,\{p,1,0\},\{p-k,1,0\}\},\left\{0,0,\frac{1}{(p^2+i \eta )},\frac{1}{((p-k)^2+i \eta )}\right\},1\right\}
