The first step is to search for the surface reference in the IDF data. For example if the error is related to surface 'W_21_4_0', search for this text. You should then find a definition for the surface, something like:
! Block 1, Zone 1, Wall - 60.843 m2 - 228.4°, Surface Area: 60.843m2
Surface:HeatTransfer, W_21_4_0, !- Surface name
Wall, 5, !- Class and Construction Name
21, !- Zone Name
ExteriorEnvironment, , !- Outside Face Environment
SunExposed, !- Sun Exposure
WindExposed, !- Wind Exposure
.5, !- View Factor to Ground
4, !- Number vertices
-6.9821656153, 4.4745082117, 0, !- Vertex 1
4.5633022044,-8.5213749335, 0, !- Vertex 2
4.5633022044,-8.5213749335, 3.5, !- Vertex 3
-6.9821656153, 4.4745082117, 3.5; !- Vertex 4
The comment at the top of this surface definition text (! Block 1, Zone 1, Wall - 60.843 m2 - 228.4°, Surface Area: 60.843m2) identifies the block, zone, and surface as described in the DesignBuilder navigator list. So the surface is in block 'Block 1' zone 'Zone 1' and has area 60.843 m2 and orientation 228.4°. This is usually enough information to identify the surface.
You can use a similar technique for identifying zones from the zone number. If the zone number is 21 for example search for 'ZONE, 21,' (without the quotes) and you will find a block of text defining the zone such as that below. At the top of the zone definition text you will see a comment something like: '! Block 1 - Zone 1' - So you know that the zone number 21 is actually Zone 1 in Block 1.
! Block 1 - Zone 1
ZONE, 21, !- Zone Name
0, !- Relative North (to building)
0, !- X Origin (M)
0, !- Y Origin (M)
0, !- Z Origin (M)
We will be improving the readability of DesignBuilder IDF output in a future version.