What application’s instructions were you looking at? There is no Look panel in Lightroom, and when I type that phrase into Google it seems to appear in a
help article for Adobe SpeedGrade, which is a discontinued application for color grading pro video clips, nothing to do with Lightroom.
Because you listed the version as 11.5, we’ll assume you’re using Lightroom Classic. For underexposed slides, the first good step is to increase the Exposure value, as shown below on my own scan of underexposed Kodachrome slide film. A second step is to try increasing the Shadows value.
View attachment 19396
If the results are not good enough, your next step is to go back to the slide scanner software and see you can get a better source scan for software to work with. For the re-scan, you want to look in the scanning software for some kind of Analog Gain or Exposure feature. Those can help produce better scan files of underexposed transparencies, making software adjustments easier. I am not familiar with Silverfast software and Plustek hardware; I use different brands.
Scans of underexposed slide film are more difficult to save, because slide film has less dynamic range than negative film. The quality of an underexposed slide film scan boosted in software always looks worse than a frame of the same image properly exposed in camera.
Also, Gamma is not a specific feature in Lightroom. However, because gamma is implemented as a midtone adjustment in many image-editing applications, if you specifically want something gamma-like you can add a midtone point in the Curves panel and drag it up. If you were advised to try adjusting gamma, the person was probably talking about pixel editors like Photoshop, not raw processors such as Lightroom or Camera Raw.
