There isn't any direct option to plot the data in a semilog plot using Curve Fitting Toolbox. The workaround is as follows:
From the curve fitting tool, once you're done with the fitting, click on File -> Generate Code to generate the MATLAB code for your fit. Then you could vary the axes properties such that 'x' values get plotted on a log scale. The following example illustrates this process:
Consider the 'x' and 'y' data as:
xData = linspace(1,10000,20);
You can click on Generate Code as shown below:
In the generated, after the plot command, you could place the following command to change the x-axis to log scale:
Hope this helps!