Make sure you have chat enabled. It's easier to browse someone who also has chat enabled.
Browse host is weird...sometimes it works & sometimes it doesn't. You might not be able to browse someone, then 10 minutes later you'll be able to.
If the person's firewalled you'll find it hard to browse them. Some people have more luck browsing from the results page, rather than from the uploads pane.

Sorry I can't give you a definitive answer here
If you use the forum search to look for browse host, you'll find heaps of old threads & maybe a few more tips. If you've just upgraded to Pro, deleting LW's preferences folder when LW's closed might help a bit (no guarantees though). Check points 2 & 3 below for instructions
Fixes for LimeWire