You are surely trying to define your nomenclature entries in this way:
\nomenclature[<prefix>]{<symbol>}{<description>}
which is the way to define nomenclature entries when using the nomencl
package.
But you're using the nomentbl
package, and the correct way to define nomenclature entries is:
\nomenclature[<prefix>]{<symbol>}{<description>}{<units>}{<dimension>}
Practically, you are missing the two last mandatory arguments.
For example, the nomencl
entry
\nomenclature[L]{$F$}{Force}
has to be rewritten as
\nomenclature[L]{$F$}{Force}{N}{ML/T$^2$}
MWE:
\documentclass{article}
\usepackage{ifthen}
\usepackage{paralist}
\usepackage{makeidx}
\usepackage{nomentbl}
\usepackage{booktabs}
\usepackage[utf8]{inputenc}
\newcommand{\abk}{\nomenclature}
\renewcommand{\nomname}{Abkuerzungsverzeichnis}
\setlength{\nomlabelwidth}{.20\hsize}
\renewcommand{\nomlabel}[1]{#1 \dotfill}
\setlength{\nomitemsep}{-\parsep}
\renewcommand\nomgroup[1]{%
\ifx#1L\relax
\large \bfseries Lateinische Formelzeichen
\fi
\ifx#1G\relax
\large \bfseries Griechische Formelzeichen
\fi
\ifx#1A\relax
\large \bfseries Abkürzungen
\fi
\ifx#1I\relax
\large \bfseries Indizes
\fi
\ifx#1K\relax
\large \bfseries Kennzahlen
\fi}
\makenomenclature
\begin{document}
\[F=m\alpha\]
\nomenclature[L]{$F$}{Force}{N}{ML/T$^2$}
\nomenclature[L]{$m$}{mass}{kg}{M}
\nomenclature[G]{$\alpha$}{acceleration}{m/s$^2$}{L/T$^2$}
\printnomenclature
\end{document}
Output:

To reproduce the error it suffices to replace the lines
\nomenclature[L]{$F$}{Force}{N}{ML/T$^2$}
\nomenclature[L]{$m$}{mass}{kg}{M}
\nomenclature[G]{$\alpha$}{acceleration}{m/s$^2$}{L/T$^2$}
with
\nomenclature[L]{$F$}{Force}
\nomenclature[L]{$m$}{mass}
\nomenclature[G]{$\alpha$}{acceleration}
Also remember that the correct command line to compile the nomenclature is:
makeindex.exe -s nomentbl.ist -t yourfile.nlg -o yourfile.nls yourfile.nlo