You would need to enter MIB OID (Object Identifier) as shown on the screen shot below, which is basically a path to the value you want to retrieve. You can try some common OIDs, such as:
1.3.6.1.2.1.1.3.0 (system uptime)
1.3.6.1.2.1.1.1.0 (system description)
1.3.6.1.2.1.1.5.0 (system host name)
Our Network Scanner comes with a few common MIB files, so you can easily pick an OID from the tree: