So, I am new to Zbrush, and recently I’ve tried exporting a mesh I made from Blender to Zbrush (obj. format) . Now when I iport the obj file, it appears kinda transparent (hollow) . Whereas when I open it in Blender, it appears fine. What am I doing wrong?
Looks like the normals are flipped/inverted (zbrush does not display backfaces). If you check the “Backface Culling” option on that Blender screenshot, you might see a similar result.
You can fix this in either program - in Zbrush you would use Tool: Display Properties: Flip
The Flip Normals button reverses the order and direction of the polygons which compose the current 3D tool. If an imported 3D object appears to be inside-out, pressing the Flip button may fix this problem. Pressing the Flip button may sometimes appear to have no effect on the object. - http://docs.pixologic.com/reference-guide/tool/zsphere/display-properties/
As you can see in the above Screenshot (8), I can’t find any thing called “Display properties” in Tool. What do I do?
Just keep scrolling down (click and drag on the tool menu when the cursor looks like two arrows facing up and down). The Tool Palette does not end with “Polypaint”, there are actually several more options underneath that your screen resolution to too small to show.
Thank you ! I found out the display properties under tool (just had to keep scrolling down), and then when I pressed Double, the problem solved. 
Double doesn’t really fix the problem though, it just hides it by rendering backfaces. Your mesh is still inside out though, and this is going to cause issues down the road. Use Flip to invert the normals.