The trouble is that the extensions on that page are built for an older version of PopClip that was more lenient about how it loaded shell scripts.
The Ruby script in the extension is marked executable but does not have the required line at the top to indicate how it should run.
The page author might be willing to update them if they are still interested in maintaing them. Possible fixes:
Remove the executable bit from the ruby file
Add #!/usr/bin/ruby to the top of the file
If you just want to try that one extension quickly, I’ve reformatted the “\em” as the following snippet which you can just select all of to install on latest PopClip:
#!/usr/bin/ruby
# #popclip
# name: \em
# identifier: org.vanmaanen.extension.texem
# description: LaTeX formatting
# requirements: [text, paste]
# after: paste-result
a = ENV['POPCLIP_TEXT']
k = ENV['POPCLIP_MODIFIER_FLAGS'].to_i
case k
when 0
b = "\\emph{" + a + "}"
when 524288
b = "\\textit{" + a + "}"
when 1048576
b = "\\textbf{" + a + "}"
when 1572864
b = "\\textbf{\\textit{" + a + "}}"
when 262144, 786432,1310720
if a[0..0] == "\\"
b = a.slice(a.index("{")+1..-2)
else
b = a
end
else
b = "{" + a + "}"
end
print b
(The above block is an extension snippet — select it then click “Install Extension” in PopClip.)