I'm not sure if you've had any luck with your issue. Unfortunately, there is no basic "open this layer" command. I'll try to outline some code I have that someone else created. If it seems a bit disorganized and choppy I apologize, I'm adapting it for your use from code we use everyday to add annotation layers to a map. So I'm hoping I can at least get you started on the right track.
'First, need to have an IFeatureWorkspace. Set your a new feature workspace from your existing workspace
pFeatureWorkspace = pWorkspace
'Create a new feature layer object
Dim pFlayer As New ESRI.ArcGIS.Carto.FeatureLayer
'Set the new object feature class to the open feature class
pFlayer.FeatureClass = pFWkSpc.OpenFeatureClass("MyLayerName")
'You can't add a feature class to an IMap object, it has to be a an FDOGraphicsLayer object so...
'Create a new feature dataset object
Dim pFeatureDataset As ESRI.ArcGIS.Geodatabase.IFeatureDataset
'set the object to the featurelayer dataset
pFeatureDataset = pFlayer.FeatureClass.FeatureDataset
'create new fdographicslayer and fdographicslayerfactory objects
Dim pFDOGrLayer As ESRI.ArcGIS.Carto.IFDOGraphicsLayer
Dim pFDOGLFactory As ESRI.ArcGIS.Carto.IFDOGraphicsLayerFactory
'The factory must be 'new'
pFDOGLFactory = New ESRI.ArcGIS.Carto.FDOGraphicsLayerFactory
'Then set the layer to the factory opengraphicslayer call with the feature workspace, feature dataset,
and string name of your layer.
pFDOGrLayer = pFDOGLFactory.OpenGraphicsLayer(pFeatureWorkspace, pFeatureDataset, "string name of layer")
'Finally add the fdographicslayer to the map.
pMap.AddLayer(pFDOGrLayer)
Hopefully this works. I haven't tested it, but like I said we use a similar process to add annotation layers so I'm ever so hopeful. Good luck and if this doesn't work for you maybe it can at least set you in the right direction with libraries and such.
