I am wondering how I can do a text/verse search in Bullinger's Figures of Speech? Also, is this a credible resource?
This may or may not be helpful, but you can search for a scripture verse in it like in many other resources.
Also, is this a credible resource?
Yes, although I wish we had an updated version that included, for example, some of the work of Berlin on parallelism, Breck's The Shape of Biblical Language or some of the form criticism patterns.
If you go to the Reading List "Logos blog and forum tips" you'll find a layout for using Bullinger:
